55 olds 88 custom. Fatman front with 69 Chevelle calipers, custom 9" rear with speedway rear caliper kit (Seville calipers with parking brake). Corvette MC from CPP and PV4 disc/disc prop valve. Bled my brains out, no pedal, bought the Motive pressure bleeder, got a decent pedal, but as soon as I start the car, pedal goes to floor. Will pump up some, but very weak and almost on the floor. Rear parking brakes are adjust really good. I move the lever 1/4" and both sides lock up the rotors, Called CPP, and they thought the MC might be bad because it sat for like 4 years, so I bought a new one. Installed it today, pressure bled all 4 corners 3 times, and it's no different. I welded some flared lines to up plug the front lines at the proportioning valve and now I have a pretty good pedal even with the car running. Pedal holds as well. So, it seems like it's the front? Not sure what I'm supposed to do or look for? No fluid leaks. Pads contacting the rotors. Could it be the prop valve? Really frustrated. Appreciate any thoughts.
At the risk of sounding obvious, are you sure you have a disk / disk MC? Is the bore size appropriate to your application?
Also, is the MC below the floor or on the firewall? IF below the floor do you have residual pressure valves installed?
No worries, yes it is correct MC I just talked to the tech at CPP. I actually just installed a new one this morning because the first one had sat in the box for like 5 years before I final installed it. He thought maybe the internal seals were bad..... Didn't make difference. He confrimed the bore size was right for the setup including booster.
MC highest point of system. Forgot to mention, that yes, the bleed valves are up top on the calipers.
Your vette MC pushes enough volume to move vette calipers of 2 Pistons in each front caliper totaling an area of 3.84 square inches. Your 69 chevelle caliper has a piston that has 4.91 square inches. That means your master will need to push more Fluid to cinch up the pads, the way it does this is to travel more. You could try a smaller bore caliper or a larger bore mc.
31Vicky, I am thinking you nailed it. I have been trying to understand all of the math and I will go out and remeasure my pedal to confirm the pedal ratio, but I think overall the front to back bias, and my leverage ratios look pretty good (assuming my pedal ratio is what I think it is). I am really confident that I bled these lines completely. I just need to figure out what MC bore to go for. 1 1/8"?
With disc brakes you don't move fluid, only a tablespoon of fluid is transferred. Drum brakes need fluid to operate the wheel cylinder, disc pads don't retract like the wheel cylinder. 1 1/8th bore will give you hard pedal without the pressure needed for discs. Did you bleed the master cylinder? You can do it on the car by cracking the fitting on the MC itself. It sounds like you have a little air bubble internal to the MC. Or not. It sounds like you have power brakes? Do you have the right MC for the booster? There is a nipple sticking out from the booster that makes contact with the piston in the MC, if you have a MC designed for manual brakes that nipple won't have anything to contact when vacuum is applied. They offer a 3/8ths plug just under an inch long as an adapter.
MC bench bled for half an hour..... Power brakes, 7" single diaphragm booster. I have the pin adapter in place behind MC. Have used the Motive Power products power bleeder, and had the wife help me pedal bleed. I've run quarts of fluid thought each caliper. Garbage pedal goes to floor. If I plug the front lines at the Prop valve, I get good pedal and it hold solid when I start the car. With the front lines hooked up and re-bled, can just barely pump the pedal (pads do grab the rotors), but if I keep pushing it will go to the floor. Don't know what else to do. Getting tired of looking at the car at this point. I understand that there's is not much volume moving in the closed system, but I would imagine the small difference in volume would change the pedal travel some. Got to try something. Right now all I have is a an expensive paperweight.......
I think when you solve the problem it will be a simple fix, the trick is finding the problem. Please post when you solve it. Maybe start removing things. Remove the booster and make manual. you already removed the front brakes and had an improvement. 31 Vicky made a very good observation. Or add residual valves. 2lb for disc. I added these to mine and made a big improvement.
I keep hearing about a style of prop valve that has a piston that travels between the front and rear sections. I have been told that when the fluid is totally gone from one section the piston will travel fully to the next and stay there so that when you reintroduce fluid to the one section it won't have any effect. I can't verify such a thing, this is what I have been told.
Brake problems on these systems that we put together out of various parts can be a real bear to fix. I think '31 Vicky is on to something with the difference in the area of the pistons in the calipers - or at least it seems logical, since the area of the Chevelle pistons is about 60 % larger than the Vette - and I wonder what the actual volume of fluid that has to be in the caliper cavity (behind the piston) to make the brakes work correctly is when comparing the Vette and Chevelle calipers. It may sound a little crazy but as an experiment you might try filling some or all of the cavity in the back of a couple Chevelle caliper pistons with epoxy (making the volume of the caliper cavity effectively smaller ) and try your experiment again - if it happened to work you would at least have an idea what needs to be done. I don't imagine you should leave these in place (if it happens to work) but it might give you some idea of a direction to go.
that is a combination valve, that also runs the brake warning light switch on OEM cars/trucks. The piston blocks one circuit and trips the warning light switch due to a detent on the piston. On GM there is a pin under a flat rubber cap that you clamp down while bleeding, and that keeps the piston at center. Some valves can be recentered by bleeding down the "good side" if that makes sense. If the valve is blocking, you will know it when you spin each wheel. If front or rears are not working with at least some amount of firmness at the pedal, that narrows it down. Also, if the piston is blocking one side, it will show up as no pressure/low flow at bleeders on the blocked circuit. .
Easiest way to tell if the proportioning valve is not centered is to hook up a test light to a power source, touch it to the center pin on the proportioning valve, if it lights up, the valve is not centered, if it doesn't light up, valve is good, look elsewhere. Sounds to me like a master cylinder problem, try gravity bleeding it, just open the bleeders on the front one at a time, wait for no air. Dumb question time, are the bleeders on the top? Have you got them switched side for side?
Just finished a 41 Chevy.... Had the same problem. One minute a good pedal... Then to the floor. I gave up on them and let it set a couple of days to work on other things. Got in the car and voila!! Good pedal!! Some times they just get a little air in the master that won't bleed out. Thinking back, we had jacked the car up and it sat over night with the tail up.... Possibly the air bubbles just got in a position to pop up into the reservoir. Yes.... They can aggravate you ! Sent from my LG-H343 using The H.A.M.B. mobile app
Is the master in the floor? Did you run residual brake valves? https://images.search.yahoo.com/yhs...=yhs-mozilla-002&hspart=mozilla&hsimp=yhs-002
With a correct brake system. I have always had success, when can't get a decent brake pedal, because air bound - Instead of bleeding one wheel at a time, bleed all 4 at once.
Master at highest point, no residual valves. Did test the prop valve with a test light & it's fine. Couple additional pieces of info. This is a 90 degree under dash assembly I got from Scotts Hot Rods. I have pedal ratio of 6:1 but that's before the bell crank. I just measured the horizontal moment on the output since of the bell crank, and it looks like I am only moving the MC piston about 3/4" of an inch once the 1/4" of free play is gone....... Now I have to figure out how to deal with that as I'm pretty sure I have that adjusted all the way in.
I just looked at the Scott's 90 degree pedal setup at their site and it looks to me like you could move the booster/master assembly (away from the firewall) with new holes or slot the existing ones and make a new bellcrank assembly extending the long arm side back a little, moving the mounting hole for the plunger rod and increasing the throw on that end. It may be that the length of that arm (center to center of the heims) is limited by the travel available in the booster. I'm sure Scott's could tell you if that might help.
OK, it's fixed!! That was it. I had adjusted the 90 degree heim joints out as far as they would go and thought the pedal was up hitting the frame but I pulled the other heim joint (front-to back attached to the pedal) and lengthened it about 3/8" and the pedal came way up (still comfortable moving foot from throttle to brake). I now get just over an inch of MC piston travel (after 1/4" free play). Pedal feels really good now even with the car running and booster engaged! I bled everything one more time just for giggles but didn't see any air. Thanks for keeping me thinking guys! This has been very frustrating and I've lost a little time on my project, but I think I learned a lot too. Will see how the brakes feel once I get the console and shifter back in......
Well, thank you for the finale! Glad you got it, although I didn't think you would have had the 90* setup. Linkage is just that: a number of 'links' that join together...like a train. Ever see all the 'free play' that gets taken up between cars on a dead startup? On a hundred car 'chain', it could add up to 10 feet. Maybe over time, the engineers/brakemen learned something. But so far, all we've eliminated is the caboose. Provokes thought...
You should try to adjust the free play down to about 1/8" or less, that will help with the pedal travel also.
I'll see if I can get any more out of it. From what I saw, the MC has a 1 1/16" travel and I measured about 1" of travel where it's at. Just happy to have brakes finally!
Didn't have any issues with the Scott's product at all. It seems really solid. Had to reinforce the firewall and make mounting brackets for it, but it wasn't too bad. Some of the newer guys paint or powder coat theirs. But otherwise they all look pretty similar. The pictures I have when it was 'visible' are on another computer. Pretty crowded under there now. Not sure this shows much...
thanks. I don't think i will have enough clearance between un underfloor MC and the exhaust and I don't like the look of the firewall mounted units. Yours looks like a cool solution. are you going to have a remote fill reservoir?
I bought the remote fill kit but haven't installed it yet. I am able to fill through the glove box opening. I removed the glove box and have shortened and motorized the door to open when the car starts. There will be a tablet mounted to the back..... If it ever need to service the master, I will remove the door. I can always add the remote fill later. At least that's the plan right now
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