So I am at my wits end on this damn thing. Here is the run down, 51 chevy ford 8" rear, all new brake hardware etc disc brake kit up front with 76 nova brakes all new 83 grand prix brake master and booster, master is new booster is original part brake pedal is 83 grand prix as well, but I've added 2" to the length from the bracket that attaches to the booster Master doesn't have any check valves in the ports, (no restrictions in the ports) I've added to the rear brake line a wilwood 10 lb check valve, and an adjustable proportioning valve. the front brake line is to the rear port on the master and the rear line is on the front port (does it matter??) I've bled them for 5 days straight, still VERY spongy pedal, sometimes NO brake when driving, and NO air coming out of the lines, and no leaks. Ideas? Questions?
Three things pop out to me. First with the master and booster you're using I ***ume they are on the firewall. If so why the residual valve you don't need it? Second you screwed up the pedal ratio by adding the two inches which will account for low pedal. Third why didn't you use the combination valve off a Nova or the G.P. instead of the adjustable porportioning valve?
The residual valve is fine - don't remove it. Are the rear brakes adjusted?? Is the booster rod adjusted for proper clearance to the m/c - it should be almost touching the m/c piston. If I understand the 2" - that was just a "spacer" of sorts - moving the booster out?? If so - it hurt nothing. If you added to the pedal length - then I agree with Phat rat - you have increased the ratio enough to allow it to feel spongy - probably be touchy as well. Most "new" systems suffer from air in the lines EVEN if the bleeding suggest there's not. You really need some force to move an air bubble in the line -especially if you have a high spot or two soem where in the system - this is one nice thing with pressure bleeders. the next thing I'd suspect is the calipers - any chance you are using the metric calipers?? Those are "low drag" calipers - these can cause a low/spongy pedal - as they are designed to retract the piston to prevent the pads from dragging - (gas crunch days) origionally they were desinged to be used with a "fast fill" m/c. SOme guy suse these without problems others don't - always made me wonder if they had the "low drag" seals or if they were replced with standard ones.
Thanks for the replys guys, the 2" was to the pedal length, not a spacer, so it looks like I screwed up the ratio, could that be the whole problem!?! I added the residual because I read that rear drum brakes need a check to keep the pads near the drums. I didn't have the factory valve block so the proportioning valve was my way around that. The calipers are for a 76 Nova, I'm not sure if they are metric or not. Seems like it may be the pedal, can I swap out the push rod? Or am I screwed??
also make sure that the larger resovoir on the master cyclinder is supplying the disc brake not the drum
Aaron, Sounds like we got more to consider here next time we start wrenching on this. Thanks for the input guys, Aaron and I have been discussing this and trying to figure this out. We'll probably be at it this weekend again. Buckle
Again - KEEP the RPV you DO want it with the drum brakes. MPB http://www.mpbrakes.com/uploads/do***ents/mcplumbingpdf.pdf has a nice website that gives lots of info. Is the larger side of your M/C plumbed to the disc brakes??? WHile yoiu may have "screwed up" the pedal ratio some - it wouldn't explain the thing "not having brakes sometimes" What I'd be wondering is what size m/c piston do you have - with your altered ratio are you sure you have full piston stroke?? Is there enough EXTRA travel in case of a partial failure - you absolutely need enough travel to bottom out the m/c (no fluid). If your ratio is screwy and travel not up to par then I could see why bleeding might suffer causing your problem. Gravity bleeding won't displace an air bubble depending on your line placement. Master Power brakes has way more info than my peanut brain can remember - here's a couple highlights: http://www.mpbrakes.com/uploads/do***ents/mcplumbingpdf.pdf http://www.mpbrakes.com/uploads/do***ents/troubleshootingpdf.pdf
I'm going to check the stroke, I agree that the longer pedal will give more mechanical advantage but it should still stop. What I don't know is if the booster pin is moving as far as it should. I'll measure it all tonight and see if that is the issue. I can tell you right now the brakes don't "come on" until my pedal is near the floor which may mean the stroke in the master is not correct.
Did you bench bleed the master? You put it in a vise and have hoses that are usually supplied in the box, pointed into the reservoirs. You then pump the master with something repeatedly until the bubbles dissapear. You can also do this in the car with some help. Loosten the brake line nuts and have someone push the pedal. Then tighten the lines, release the pedal, and do it again until free of bubbles. The residual valves on drum masters are to keep some pressure on the cup seals so they don't leak. On Mopars, they stopped using them in the '70s, and have a cupped expander in the wheel cylinders to accomplish this task. Don't know about other makes, though.
The problem with your thinking on that is that the pedal can bottom out on the floor before you're even close to full stroke. The ratio should be around 5-1 this means the pedal has to move 5" for 1" of rod travel in the master. Lets say you have a ratio of 8-1 now because of lengthening the pedal, now the pedal must move 8" for that 1" of stroke. Is the master completely stock or did you remove something? If it's stock take the adjustable proportioning valve off and use a combination valve from an S-10 or look for an 83 GP. I know others disagree but the check valve for the rear isn't needed and many have had a problem with the Wildwood valves. I doubt that the booster rod is the culprit as you're using a matched set-up with the master and booster
The master is stock, but would the proporting valve affect the front circut? This is something I never really understood. If the front and rear brake line circuts are separate (fail safe) would anything added to the rear lines affect the front lines? Such as would having any kind of check valve or proportiong valve on the drums affect the braking of the discs? I understand the locking up of the drums, I wish I had that problem Also based on the 5:1 or now 8:1 ratio if the rod was adjusted out or "shimmed" with washers wouldn't that help? If it needs to go 8" but hits the floor at 7, then I'm not pushing the piston far enough, right?? BTW Thanks for everyones help so far I don't feel so lost now!
Also I should note, the brake "feel" has been the same with or without the check valve and proportioning valve. I started this whole mess with no valves, just the master and lines to the brakes. Then I blocked off the rear brakes, then I added the check and proportioning valves. During all of this the brakes have had the same soft gradually stopping feel.
Well I'm thinking your not afraid to cut the pedal based on the fact that you did to start with. So why not cut it again and play with different ratios? You could also play with the angle of the pedal. Your building a hot rod so build the pedal to suit you and your car. The pedal could very well be the problem. I would hesitate to guess how many times my pedal has been cut as i had a similar problem with too much ratio to begin with. I know the feeling of frustration with brakes as I fought a problem for a few months when I switched boosters. I even switched from 4 whl drum to 4 whl disc during this time and still had the same problem. I finally found that the booster was bad out of the box, but didn't give the hard pedal feel. Customer service from MP Brakes at the time is the reason I would never again use any of their parts
I think both Phat Rat and Hemirambler have you on the right track. Id venture to say that you increased the length of the pedal (think like adding a cheater bar to get stuck lug nuts off a rim) hence bringing two new factors into your system: 1. You made the leverage of the pedal require less, hence a spongy feel. 2. You possibly also made it to where you cannot get full travel of the pedal. A couple of other factors to consider: The wilwood proporting valve and the residual valve will often be AN fittings. When you couple them with standard brake line fittings, you have to use the sealer junk (looks pink and dry on the adapters willwod provides, you can get white liquid teflon sealer from Napa, preferable to just get the proper adapter fittings though!). This can often lead to leaks, which will also lead to a spongy pedal. Your best bet to tackle the problem will be to take each little section of the brake system at a time, and fix each area. Id start with the pedal (since that is where you seem to have the most issues) and work your way out to each wheel. I know I never want to pull by brake system apart on my 35!!!!!!! It took me over a month to work out all the bugs and issues.
Ok well to close the loop, I got home and checked out the master and pedal. The booster has a plastic pin that engages the master, I pulled it out and put a 1/2" thick nut about the same size as the pin in the bore then re-installed the pin and master. The brakes WORK! Yes the pedal is sensative and if you step on it, the tires squeal but the WORK! So now I have to see if that plastic pin comes in different lengths. Thanks to Phatrat, Hemirambler, Buckle, Dirk, everyone for giving me ideas and calming me down!
I, too, cut and lengthened a pedal to get the clearance/movement that I needed. I also moved the pin to compensate for the increased leverage/travel. If your pin (which attaches to the master pushrod) is moveable (ie: bolt on), move it down a bit, and you could improve the whole as well. Cosmo
Sounds great. One caution though. Have you driven the car any amount of miles yet this way? The reason I ask is that I wonder if possibly the brakes could be part way on at all times. If so driving the car 30-40 mi would tell you as they would heat up and cause a problem. I have a hard time believing that the matched booster and master would be off like that. You did state that the booster and master were both 83 GP right? I think in essence what you have done is account for excessive pedal travel doing this. The pedal travel was caused by incorrect pedal ratio. Of course I could also be 100% wrong, but i don't believe so. I've looked at those rods in the past and they seem to be pretty consistant in lenght in the GM line at least. I don't remember seeing any difference and certainly nothing near a 1/2". Another way of doing this would be if the rod going into the booster from the pedal is adjustable. You want about 1/8" of free pedal, by this I mean travel before you feel resistance
You nailed it, the spacer was too big, and the brakes dragged but I wanted to check that that was the problem. I measured the stack-up and it was about 3/16" out, so I replaced the nut with a couple of washers. I'll run it some more and see if it's still dragging. At least at this point it's a matter of tuning the spacer to get the right length. I can deal with a sensitive pedal, after all I'm a sensative guy
I didnt read all of the replies to see if anyone answered this yet so I'll throw in 2 cents worth. The rear shoes dont need a residual valve to keep the shoes close to the drums, that is accomplished by the adjusters. If they are self adjusting then no problem. If not then an occasional adjustment is in order.
All you're really doing is using a crutch to compensate for a problem elsewhere. I think the problem your covering up now is too much pedal travel due to the wrong pedal ratio. Work on that and I think a lot of the problem, if not all, will be fixed. I remember the frustration I went through when I was having a similar problem. I even tried something similar to what you're doing
The factory has employed (as far as I know) TWO methods to keep wheels cylinders from ****ING AIR when the pistons retract - THAT is what the 10 # RPV is for. IT's NOT about sefl adjusting shoes or non self adjusting shoes. If you don't use a RPV you would want to employ the "other" method - specifically use wheel cylinders with LIP SPREADERS (basically a formed washer) that does a similar thing as the RPV -preventing air from being ****ed in, but does it by forcing the cup seals to remain in place. I think the RPV's became more popular because they helped "reduce" the "slack" in your hydraulic system - not overcoming the return springs. Most "old" M/Cs had RPV's BUILT IN them - that's why so many guys swear "you don't need them thar new fangled thangs" - Problem is that they never realized they had been running them for years. The Combination M/C's used the "duck bill" version in the drum side exit port _look carefully beneath the tube seat and you'll see it. The old "fruit jar" (single reservoir) type M/Cs used the "tin washer with the flap valve of "rubber". It's the cupped tin disc thingy in the rear of your single reservoir M/C. The additional confusion is the 2psi RPV usd with disc brakes. Most often employed when your M/c is mounted BELOW your caliper.
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