I have fitted a set of twin piston calipers (PBR) and big rotors to my 57. It has a relatively new master cylinder and 7 inch booster. Since fitting this set up I can't get pedal pressure. The old calipers and rotors were circa 1970 (HQ for any Aussies reading) and had plenty of pedal. I have been told by a few people it must still have air in the lines but I have put 3 bottles of fluid through the system trying to bleed them. It has also been suggested I need a bigger bore master cylinder. Any Ideas?
sounds like you need more volume of fluid to your front calipers, ie bigger bore or stroke in your mastercylinder, go 1/16 inch bigger would most likely work
Could be one of two things: either not enough volume in the master cylinder, in which case a larger bore would be the solution. If you are able to stop currently, but have a spongy pedal, it would be more likely a pressure issue, in which case you would want a smaller bore.
My achy breaky heart I just don't think it'd understand And if you tell my heart My achy breaky heart He might blow up and kill this man -Billy Ray Cyrus Going back under my rock now.
I'm going with bigger bore. the area you are now trying to fill with additional fluid is now larger than the old one.
My guess is you still have air in it, but it's probably got a leak in the system that prevents you from getting all the air out. Or it's possible the bleeders valves are not "up" all the way, preventing you getting the air out. The change in volume is not going to be nearly enough to prevent you getting the brakes to work at all. I swapped my 55 to wilwood 4 piston calipers several years ago, then to a smaller master cylinder to increase stopping power, and the pedal is still way up.
You better do some math before you go buying any other master cylinder. I believe the dual piston PBRs are smaller in piston area than the single piston big GM caliper(i think thats what you have). I have seen a few different sizes on the PBRs. If the piston area is smaller you need more pressure to make them work(as in a smaller bore master).
Thanks for all the input, except yours takid455 (now I can't get that ****** song out of my head). The pedal is spongy but if I dent the floor pan with the pedal I have enough brakes to line lock it in the shed. Last night a mate told me he once had to put 5 bottles through before he got all the air out. He said he ended up doing what he called a gravity bleed. Apparently you open both bleeders and let gravity do the rest. He said to crack both bleeders, go and get a beer, come back and top up the reservoir, go and get another beer and so on until I've put 2 bottles through. My mates only warning was that by the time you get the second bottle in you are drunk as a monkey and might forget to close the bleeders.
make sure the bleeders are facing up! easy mistake to have them pointing down. (ask how i know) youll never get all the air out if theyre pointing down
Did you bench bleed the master before you mounted the lines? If not, air may be trapped in the MC bore no matter how much fluid you pump thru the system.
Hi Staleg The master cylinder and lines were existing. They have not been removed. The only new components were the calipers and flexible lines. I don't exactly understand what a bench bleed is but no I didn't do one. I have changed plenty of calipers and master cylinders before and never had this problem which is why I am leaning toward a different bore master cylinder.
Before bleeding, tighten all ******s. Try to pump up some pressure in the system and then keep the pressure on for a while. After that, check for leaks. Leaks will **** air into the system every time you release the pedal. If it's not possible to build up pressure, I would try a bigger bore master. You probably need 1 or 1 1/8" bore. What do you have now? When you bleed the calipers, watch how the bubbles in the hose from the caliper acts when you release the pedal. They should remain in the same spot and not move back towards the caliper. If they move back toward the caliper, you have a leak in the bleeding equipment, og maybe the ****** is loosend too much and **** air p*** the ****** threads.
It's hard to tell from the picture, but it looks like the bleeder valves might be off to the side a ways, allowing an air bubble to stay in the top of the calipers. You could see if you can remove the caliper mounting bolts, and hold the caliper so the bleeder is at the very top, while you bleed the brakes. If you can't keep the pads on the rotor while doing this, you may have to remove the calipers completely and put something in between the pads, the same thickness as the rotor (such as a piece of wood) while you bleed them. But try the gravity trick first.
Thanks Staleg and Squirrel. I've checked for leaks already but couldn't find any. I will have another shot at it this arvo after work. I might try removing the top caliper bolt and laying them forward so the ****** is as far up as possible and gravity bleed them. If I do my next post might make no sense as I will follow my mates instructions to the letter, top up/beer, top up/beer, top up/beer until pissed.
Still scratching my head. Did the gravity feed bit (now got a hangover) and will bet my left nut there's no air in the system. I laid the calipers forward so the bleeders were at the very top (pic) and didn't even attach hose to the bleeder. Just let it run down the caliper into a bottle. I've had some tell me I probably need a larger bore master and some say smaller. Can anyone point me in the right direction?
Had same problem when changing to larger rotors and big piston calipers. Front brakes would not apply hard,found I needed a master cylinder with more volume to the front calipers. Ya I poured 2 cans of fluid through and bled every way but no front brakes. changed MC , got brakes right away.
When you say you "cant get any pedal pressure", does the pedal go to the floor or is it just soft? If it's going to the floor you might have an air lock (anyone you can borrow a vacuum bleed or pressure bleed setup from?) if not that could mean the master cylinder is too small (unless it's smaller bore than 7/8" / 22mm I find that unlikely) but could also be caused by the booster pushrod being adjusted too short, it should just barely clear the master cylinder piston when everything is relaxed. If that is all good, do you have a way to check the pressure at the calipers? If you don't have the tools you can make an adapter fitting out of a bleeder screw (through bore it and make a way to attach a high pressure gauge to it, borrow the gauge if you can). If the pressure seems reasonable (I don't know what is ideal for that particular combo, 1100 Psi, 1400 Psi???) you might have an issue with pedal ratio (too much), did you get a new pedal with the booster / master combo? Of course that ***umes you have vacuum for the booster, have you tried it with vacuum (started the engine, vacuum pump)? How much vacuum does your engine produce? Is the booster worth a darn (bad new parts do happen from time to time)? How are the brakes at the other end of the car? Unlikely that the rear is consuming all of the movement with some sort of issue, but not impossible (as long as the rear isn't full of air, it shouldn't be and issue for this) Is all the plumbing new? If the line to the rear is old (front stuff in the pics at least looks new) it might be corroded, or have some schmutz in it that is caussing an issue. If the rear brake hose is old enough it could be balooning, contributing to this problem (unless it's braided Teflon, like the front appears to be). The last thing that comes to mind is, are you using silicone brake fluid? I have not, but everyone I've ever spoken with in person that has, tells me that the brakes were always spongy with it. Problem went away with fresh "normal" brake fluid. Good luck, with as much as has been changed in that system it could be a PITA to find what it is.
Still having issues with my hunk o' junk. I want to post a video showing the problems. Anyone know how?
subscribed. I have a 57 Chev with HQ-Z rotors ,a Torana booster with HQ style master cylinder and the stock rear drums. my pedal still feels a little spongy too....thought a proportioning valve may help but haven't got that far yet. Theres some good ideas here of things to check.I'm interested to know how you solve your problem. Don't want to hijack your thread tho oZ 57....
My problem is sometimes the pedal reacts like it has no booster and some times it reacts like there is a booster. I can drive 20 Km and have 5 different pedal pressures. I'ts driving me insane. Sometimes the brake feels like a 200 series landcruiser, sometimes like an FJ. If I get an answer I will be happy to share!
I feel your pain, i hate brake problems. You last post has thrown me completely, but i'll say what i was going to anyway. So nothings changed except the calipers, right? When swapping the calipers did the sytsem drain down or have you allowed it to run low on fluid in the bleeding process? If that's the case then you may well have introduced air to the mc and you're looking at 'bench bleeding'. The mc appears to point upwards in the pic. If you jack up the rear so that it is pointing down (slightly) and bleed the **** out of it you'll most likely byp*** the 'bench bleeding' requirement. Otherwise, look up bench bleeding and do it. If there's air in the mc with no way out then no amount of bleeding will fix it. With the sytem bled properly, if there's still no pedal it looks like a volume issue. Your pedal has long travel so the mc is seeing lots of stroke so is possibly in danger of bottoming out. Getting more stroke by adjusting the pedal ratio isn't going to help then. So more volume will come from a mc swap. A mere 1/16" adds a considerable amount (% - work it out!) to the piston area, and thus the volume. If this gets you a nice pedal you may not have effective brakes due to reduction in pressure (bigger mc dia = lower pressure for given input). A 7" booster is fairly ineffective. An 8" dual isn't that much bigger but has consdierably more power, so this swap can help you get the pressure up. A brake line pressue gauge is your friend in this - helps establish exactly what's going on. Hope this helps Chris
did you mount the calipers "upside down" bleeders screws pointing down? if so switch right side to left side and rebleed. Godspeed MrC.
check the pushrod from the pedal to the master may need to be lenghted also when i added discs to the rear of my wagon we couldn't gey a pedal o matter how much we bleed them till we removed the caliper from the axle wedged the brake pads and then held the caliper straight up while we bleed the brakes and in no time had a pedal
Thanks HAPPYDAZE that is very good info. I am thinking the booster is faulty due to the inconsistency of the problem. It seems like the only variable should be vac***. Do you know if there is anything in the booster (valve/diaphragm) that could fail and cause this? mrconcdid I have checked all that and posted some pics earlier. I don't think its a bleeding problem now. 59 brook same as above. I have great pedal pressure sometimes and sometimes it feels like I am going to put my foot through the floor to pull up.
Sorry bout that, my phone didnt show any pics. I used a 3/4 chevy truck master cyl. before it has a considerable larger bore than p***enger cars. Change the check valve on the booster, it may be leaking, where are you pulling your va***me from carb or manifold? do you have a large cam? Just general things to check Godspeed MrC.
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