I did a disc brake conversion on my FORD and have no rear brakes. Brake is hard and does not drop with no vacuum. When vacuum is applied the pedal goes to the floor. I cannot get any air out of the system and have very low pressure to the rear. I can feel the rod in the valve push forward when power is added. I have read that bleeding this system is a pain. I see there is a tool that is offered to prevent the valve from opening when bleeding. I have also read to use a c-clamp on the pin to prevent it from moving. What have you done? Or do I have a bad valve?
did you bench bleed the master cylinder? i would start there, then open the front bleeders and push the pedal to the floor, then open the rears and let all 4 gravity bleed. when they are all dripping close them and bleed normally.
A picture of your valve/setup would be helpful, as some of your statements are confusing. One, the brakes should be harder with no vacuum ***ist. Two, the "rod" may be the metering valve on your combo (?) valve, which limits initial front pressure, not proportions rear. Again, pictures needed. 'Course, I'm going to recommend removing any factory combo/proportioning valving and just use an adjustable prop valve, but a I'd still like to see what you have.
Bench bleed the master, can be done on the car too with a helper. Make sure the rear shoes are adjusted, I'll make them a bit too tight and back them off after the bleeding. It sounds like the rod from the pedal may be a bit too short.
Some time ago, I saw a post on how to center the combination valve after loosing all fluid in one bowl of m/c. There was a procedure to recenter the valve. I meant to print it out, but didn't and have never been able to find it again. Anybody remember that thread? Or how to recenter a combination valve?
Never had to recenter a GM prop valve, it always did it itself when I added fluid after repairing the failed brake parts. No rear brakes I agree on bench bleeding the master and if this is a bunch of different parts put together make sure the master cylinder rod is retracting all the way when the pedal is up. One time I had a collapsed rear hose that blocked off all fluid, looked like a clogged artery. I couldnt believe it but I cut it open and sure enough it was clogged..I am ***uming you have all new brake parts.
The master was bleed on the bench and I do have flow to all four wheels. The valve is a stock none adjustable chevy style from 1975 to about 81. What I see most conversion kits use this valve. Since it is disc in the front and drums in the rear the valve prevents flow to the rear until the front build pressure. Drums work on about 400PSI and the disc work on 700 PSI. When the rod in the valve starts to open it is preventing flow to the rear. I have very low flow to the rear that does not push the wheel cylinders. I have read around and this seems to be a problem when a complete new system is installed and getting air out of the valve and rear are near impossible due to valve closing the rear when bleeding. So either I have air or the valve is bad. Already tried another master. Double checked by gap from the booster to the master. I do not think the valve is bad because I have no pedal. What I understand is system must be bleed with the vacuum applied. And everything is new and the sytem was a kit.
I am very familiar with the style of valve you are talking about, I have put them on dozens of cars..however I normally use used GM stuff rather than new aftermarket shiny ones which I have read about commonly having problems. Never had to bleed one of these valves with vacuum on it either. I would not count out the shiny $25 valve as being NG out of the box, they do not have a good reputation, especially for leaking. My favorite disc/drum proportioning valve to use is one from a 78-88 monte carlo, I keep a bunch on hand and use them instead of the aftermarket stuff.
That's not how the valve works. Metering (which I think your talking about ?) holds off the front discs until the rear shoes contact the drums, or about 100 MC psi. Proportioning delivers a percentage of MC pressure to the rears once the crack or knee point has been reached, because discs do require higher pressure than drums, as you stated. Ditch the damm 40 + year old GM valve, get an adjustable rear prop valve, and then maybe we can make sense of what your really problem is.
I just restored and sold a 69 Camaro pace car for the January '15 BJ auction in Scottsdale. Same valve you are using. I went through 3 bad stand off valves and 2 bad proportioning valves before replacing with good parts (actually cleaned up/restored and reused original valves and gave up on the gold iridium plated ones. I was bleeding with power vacuum bleeder and the parts were supposed to be decent quality restoration parts, long story short and a lot of frustration later it came down to inferior parts. Not sure if that's your boggle but I hope it helps.
I'm using a CPP proportioning valve in my Ford, a stock car piece. It has the rear bias adjustment, which I may have to fiddle with once it hits the road. But I wanted something simple that I was familiar with. You may have better luck with an older-simpler Mustang style proportioning valve.
Pretty sure this is correct info. It is important to understand that an adjustable valve you suggest is a reduction valve. It is either open and full pressure or reduced pressure with each turn of the handle.
Not exactly. Full MC pressure does p*** through the valve, until the "crack point" or "knee" has been reached, which the adjustment knob ( normally about 10 turns lock to lock) moves from about 100 to around 1200 psi. From that point on, as the MC input pressure increases, a percentage of MC psi goes to the rears, like 27, 43, 59% etc. The percentage is locked in, but the pressure up to this point is adjustable.
To answer your question on the proportioning valve, I screw the switch out and install a bolt with the same threads, I drill the bolt and put a pin in the hole of the bolt, when I screw this into the proportioning valve, I can bleed the system without worrying about the valve closing off one side.
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