Technical What's the correct brake bleeding sequence for GM master with bleeders?

This type of master cylinder with bleeders on it:



Is it bench bleed the MC, connect the lines, MC bleeders, then the four wheels? Or bleed the MC, connect the lines, bleed the wheels, then MC bleeders?

I'm having a hell of a time bleeding my brakes. I've bled brakes before without any more than the usual aggravation, but I've never dealt with one of these GM masters with bleeders on it before. I feel like I'm chasing my tail bleeding this system, like I'm doing it wrong.

I bled the MC by looping the output ports back to the reservoir until no more bubbles. I left the MC in the car, but lowered the front end until the MC was level, to ensure there was no high spot for air to hide in.

I plugged the ports, and bled the MC bleeders. At this point, I had a good hard pedal, car running or not (it's a power MC, this comes up later).

I unplugged each port, and connected the lines as quickly as I could. Still, I assume some air gets in while doing so.

I bled the four corners (RR, LR, RF, LF) until no more bubbles.

With the car off, I have a hard pedal. It goes about 1/3 travel, then stops. With effort, and both feet, I can push it a bit more, to maybe 1/2 travel, but that feels like flex, not air.

When I start the engine, I have no pedal. It goes straight to the floor with hardly any effort.

I'm focusing on the MC bleeders because they're unfamiliar. That could be a mistake. It seems like bleeding them before the wheels would help get out any air introduced by connecting the lines, so should be done first, before the wheels. But the usual bleeding sequence is to work from the furthest wheel to the closest, so looking at it that way, the MC bleeders should be last.

 

Does it have a power/vacuum booster? - is it working correctly?

 

Never seen a master with those bleeders before. Look useful! I'm thinking they might make bench bleeding a redundant process - but i don't really know!

Sounds like you've done everything and more that should be done. Booster is where i think I'd be looking. The engaged brake pedal normally drops some when the engine is started, but not that much!

Has this setup ever been in operation before? If not, or if things have been changed, maybe the master bore is on the small size and what you're experiencing is related to that, ie excessive pedal travel.

Chris

 

Try disconnecting (and plugging up) the vaccuum feed to the booster, so you essentially have manual brakes. Bleed the system, and then try them out. If the problem re-ocurs when the vac feed is again connected , I would be looking at the booster.

 

What's the pedal ratio?
Power brakes want a 3-4:1 ratio.
Manual brakes want a 6:1 ratio.

Swapping the 2 around gives you a goofy pedal. Wth a 4:1 on manual brakes the foot effort is too hard and caliper pressure is low. With a 6:1 and a booster you can have too little pedal effort and push the pedal to the floor and caliper pressures so high the calipers bend/flex & hoses get stressed
Similar to what you are describing- you have Normal brakes until the booster gets energized by vacuum

 

The bleeders on the master are a great addition. Especially when swapping just a master on a working system that has remained closed. The air bubble comes out right there instead of classing the air out to the calipers. Calipers with rusty bleeders that round off or beak off don't need to be touched. Then it eliminates an ass ache or what turns into a caliper swap too.

Generally, the lines can be bled at the fittings with the appropriate technique just the same as the bleeders accomplish. Slow steady pressure not pump/build/release. With those bleeders it's the standard technique.

You could just ignore them bleeders and treat it like every other brake work you've ever done

 

Pedal arm pivot to pushrod is about 3" - 3.5". Pushrod to pedal is about 12" - 12.5". So the ratio is ok. This part didn't change, though I didn't build this originally, so thanks for the reminder to recheck everything.




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Yes, there's a vacuum booster. Seems to be working correctly.



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This setup was in operation. The master, distribution block, calipers, and cylinders are unchanged. I ran new hard lines and rubber hoses.

The old hard lines were a mess. They appear to have been created from the pre-cut lengths to avoid needing a flaring tool, so lots of adapters and couplers instead of the right tubing nuts. They appear to have then been bent by hand, without a tubing bender, using loops and S shapes to use up the extra length. And, unsurprisingly, some of those bends crimped the tube partially closed in a couple of places. The new lines are CuNiFer (Nicop), cut, bent, and flared to fit neatly, with the right size tubing nuts, and no adapters or couplers.

With the old lines, the brakes worked, but there was a brief jerk in the steering wheel when they applied. My theory is that the crimps in the lines were impeding fluid flow, so the calipers were grabbing at different times as the pedal depressed. Once pressure built up and the pads were engaged, braking was ok.





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I'm thinking I can't ignore them. Air above the line would still potentially interfere. Bleeding them here again tonight to see if it helps.



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So tonight, after some time to step back, think about it a bit, and regroup, I decided to divide the problem, hoping to conquer it. I started by removing the front lines at the distribution block, and plugging the ports there. That left the two lines from the master to the block, and the rear line connected.

With the engine running, it's like stepping on a brick. The pedal moves about 1/3 of total travel, and stops. I can hear it taking up slack in the rear drum cylinders as it moves. But after that, its rock solid. I'm now pretty sure that the master is ok, along with the booster.

I then reconnected and bled the right front wheel. With the engine running, the pedal now goes pretty easily to about 1/2 travel, then it's like stepping on a tennis ball. There is resistance to further movement, bit it's a bit squishy. With both feet and some effort, I can put the pedal to the floor.

I reconnected and bled the left front. Engine running, it's worse still. The pedal goes 2/3 down easily, then it's squishy like an old tennis ball. With only moderate effort and one foot, I can push it all the way down.

A couple of theories.

The rubber lines (new) are expanding under pressure. I know stainless hoses are used because they don't expand under pressure, but is the difference pronounced enough to be worth it? (I'm not interested in how they look.) I'll need to get my wife to come step on the pedal so I can watch the hoses, to see if I can see anything. I can try putting the old rubber hoses back on to see if that changes anything.

Possibly the old crimped lines were acting as an uncalibrated residual pressure valve. I could add 2 lbs. residual valves. I don't think I should, though.

The caliper pistons are currently all the way in, with new pads installed. The pads are pretty tight to the rotors, so the pistons aren't moving. I considered pulling the calipers and inserting some 2x4 scraps to allow the pistons to come out a bit, then bleeding again, in case there's some way the piston is trapping an air bubble that might release if the piston could move.

Maybe I'm just expecting too much? Mid 70s GM passenger car power brakes were pretty soft and easy to push, so granny could drive her Buick down to the corner store. Maybe I just need to put the wheels back on and go test drive it to see how it stops and stop obsessing over the pedal feel while it's up on jack stands.





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Are we sure the bleeders on the calipers are upright? As in right at the top, not merely toward the top. Might need to remove the bolts and rotate them some for bleeding?

I'm not buying your soft, granny brakes - that'll be a combo of pedal ratio and booster size that makes those brakes easy. The stopping power needed at the wheel will remain the same, more or less.

New pads huh? Did the pistons retract fairly easily to allow the new pads to be installed? Crud or corrosion on the pistons (which has been allowed to form when the pistons have been out to compensate for the wear in the old pads) can cause problems. The seals in the caliper are the outer edges of the caliper so are easily affected.

Chris

 

The master cylinder in question looks like an early GM, although some Chrysler products may also have used similar, and possibly Bendix. I am pretty sure the reason for the bleeders was because this design's outlet ports are located lower off the main bore, allowing air to be trapped and not easily bled through the ports. Enlarging the bore while using the same casting (common practice) only made bleeding more problematic, hence the high located bleeders.
When bleeding any system, going around or alternating two or more times on all bleeders will give the best results. Slow, steady and full pedal applies with or without a pressure bladder is key to a good bleed.

 

Yes, the bleeders are at the top. I should have mentioned that.



Agreed on the stopping power at the wheel. That's going to be a function of line pressure. I'm just looking at a 4:1 pedal ratio, on the high end of what's recommended for a power brake, and a pretty big booster. These parts all look like what I remember from my first car, a '73 Buick LeSabre. That boat stopped pretty good, and I don't remember there being much pedal effort.

The pistons weren't out much, and retracted easily with a C clamp. There could be crud in them, I guess. Having not taken them apart, I can't say that there isn't. There was crud at the bottom of the MC reservoir, which I cleaned out, and the fluid was dark with age or crud or both. I've run 5 x 32 oz. bottles of fluid through, though, so anything that was going to flush out has done so by now.




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I've been around so many times I'm getting dizzy.

From Looking around online, this MC design wasn't used for long, only a few years by GM. It does seem nice to be able to bleed the top end of the system.




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Tonight I went back to just the rear lines connected, and back to a solid pedal. I left the left front plugged at the distribution block, reconnected and capped the right front hard line, and confirmed that I still have a solid pedal. So the distribution block is ok. I had read somewhere that they can fail internally and cause problems.

I had the idea of testing the rubber hose next. I need to get a 7/16" bolt and nut and some new copper washers to plug the banjo fitting. I tried with a 3/8" bolt I had on hand, with the old washers, but couldn't get it to seal.

I put the old hose back on, thinking I'd eliminate it being the hose that way. It seems about the same, so I'm thinking the new hose is ok. It also seems unlikely that I'd get two new hoses that were both bad. Probably.

With the hose attached, I took the caliper off and rotated it in every possible orientation, just in case there was somehow an air pocket trapped in it. I didn't expect much from this, and it didn't seem to help any. Bleeding it afterward, no bubbles came out of the bleeder.

Last, I took out the new pads and put the old ones back in. This allowed the piston to come out a bit (1/4" or so) from its fully retracted state. I bled it again, and didn't get any bubbles. Pedal seems about the same. It goes down about 1/2 way easy, another 1/4 of the way with some effort, and almost all the way to the floor if I push on it. It does stop shy of the floor by a bit, but that's with the left front plugged. That's similar to what I saw last night, when testing the fronts.

I had to stop here. I've either worn out the bleeder screw on the right front caliper or damaged the seat. The bleeder no longer seals, it's seeping fluid under pressure. I'll swap the bleeder tomorrow.




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Recently did a disc brake conversion on a Tri-5 Chevy and had problems getting the front brakes to bleed out even with my vacuum bleeder. Used the old pump them up and tap on the calipers with a small hammer as I opened the bleeder and the bubbles came out and had good brakes. Air can be stubborn sometimes.

 

Well, I've narrowed it down to the calipers. I put the new hose back on the right (left is still plugged at the distribution block) and plugged the banjo end with a bolt:



Nice hard pedal.

Reconnected the caliper, and bled an entire 32oz bottle of fluid through it. If I then wait a few minutes, pump the pedal, and bleed it again, I get bubbles. If I squeeze the piston in with a C clamp and the bleeder open, I get bubbles. And the new bleeder won't seal either, so I must have damaged the seat by opening and closing so many times.

New calipers (remanufactured) with hardware are $20 each, on sale for $18. They'll be at the NAPA tomorrow.




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Well, it's not the calipers. Put two new (reman) calipers on, bled it again, and it's no better than it was. So, having replaced everything else, next up is the master cylinder. Could be leaking internally. It'll move fluid ok, so it bleeds ok, but won't build line pressure. Not sure how to diagnose an internal seal leak other than a li r pressure gauge (don't have), or swapping it out. Back to Napa tomorrow.




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Brake fluid isn't too hard to outsmart.

Your initial steering wheel jerk was more than likely a mechanical issue not hydraulic. I'd suspect sticky slides or Pistons.

From your descriptions it sounds like the system is now sucking air into it when at rest.. Could be anything anywhere you've touched.

 

Replaced the master. Had to re do the flares on the two connections to the master, they wouldn't seal up on the new one. Bled everything again. Still using the checkvalve method, but I added a remote camera with my laptop sitting on the passenger seat, so I could see the bleeders. Pumped the pedal until I could see five pumps in a row with no bubbles.

I'm not convinced that I'm done here yet, but it's definitely better than it was. Pedal effort still seems too soft, and there's too much travel before I feel the brakes start to grab. But the pedal no longer goes to the floor, and the tires are now the limit on braking. It'll lock the wheels on hard braking.

With the Wisconsin 100 coming up, I'm calling this good enough for now. I'm going to put some miles on it, then I'll bleed it again next week to see if anything comes out. It's not leaking anywhere.



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I'm pretty sure the steering wheel jerk was hydraulic. There were crimps in the line, which would restrict fluid flow. That problem is definitely solved now though, whatever it was.




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This sounds good. Better that way than too hard a pedal and less effective braking. If the sponginess has gone it sounds like you've sorted any trapped air issues, if indeed there ever were any! Seems to me it might be time to consider a small master diameter increase - a small change makes a noticeable difference - do the math on the area! Do we know what the existing diameter is?
Chris

 

If a kinked line caused a restriction in fluid flow under pressure the fluid would face an impossible restriction on return.

 

Brake fluid may not be, but people can be tricky. I'm adding a new wrinkle to the story.

I was out on a local cruise and barbecue today. Mostly non H.A.M.B. friendly cars, it's just good people enjoying a drive in the countryside, a few nice stops along the way, and a huge lunch party at the end.

Couple of early 70s Chevelles in the pack, plus a 52 Chevy pickup with a setup similar to mine. So I borrowed some seat time to compare pedal feel and travel, and yeah, something is still not right with mine. It's not soft, it doesn't pump up like it would if there was air in the lines, but it has excessive pedal travel.

Talking to one guy, who's business is selling muscle car parts, he's 99% sure that what I have:



is a drum/drum master. Also, that what I had, with the bleeders on it, was a drum/drum master. He's pretty sure that the disk/drum master would look like:



which is what was on both of the Chevelles I looked at. The truck had a different one, he says he's seen Chevy use both styles on disk/drum cars, so it was probably just two suppliers making compatible parts.

So, possibly I've been trying to make mismatched parts work. One of my troubleshooting rules is "always suspect previous work", and with the way the original lines were done, I'm now suspecting that he may be right.

Guess what I get to do again...




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That top one looks like a disk/drum master and the bottom one looks like a generic corvette master. I would do some more research.

 

I'm not seeing a difference in the master reservoir sizes providing you with a fix for where you are currently. The principal difference between those masters is that the drum element has a smaller reservoir as it doesn't require additional fluid to compensate for the wear in the friction material, as a disc brake does. The different capacity makes no difference right now in terms of the braking performance. If you have too much travel and there aren't any other issues as the brakes work well now, you could improve the pedal travel with a small increase in master diameter. Do I hear an echo? ha ha.

You've still not yet disclosed the existing master diameter. You have a 4:1 pedal ratio and a booster of some kind. My guess is that the master is 1" but 1.125" is probably what you need. You already know there's another master cylinder in your near future, but changing the reservoir size isn't going to make any difference.

Chris

Chris

 

Your master with the bleeders, and the NAPA with the red and blue plugs are disc/drum, with the primary portion feeding the rears. The last picture is a Corvette style disc/disc that can be also be used on drum/drum and disc/drum.

 

The original master bore is unknown. The current (Napa p/n P36280) bore is 1 1/8". The other master (picture is of Napa p/n M1974) is 1 1/8" also, so yeah, that's not changing anything. Agreed, reservoir size isn't changing anything that matters here.

Called Napa yesterday. They're normally helpful at the local shop. They couldn't tell me anything more about the p36280 master, but came up with the p/n m1974 master as a disk/drum '70 Chevelle. That conversation wasn't especially helpful though, as the guy wanted to know what the old one looked like, one bail or two.

I'm not thrilled with pulling it apart again, but I think that's going to have to happen. I also need to verify the pushrod length while I'm in there. I forgot to do that when I put this one on.

Gotta pick up some more brake fluid first. I hope they've restocked the shelf.




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What makes it disk/disk? The larger rear reservoir?



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