A couple of years ago we bought a '58 Nomad that the previous owner had installed a front disc brake conversion on from The Right Stuff. All of a sudden we lost brakes (pedal to the floor) in it and this issue has me stumped. There's a Tech Question on their web-site with the exact same problem with no real answer other that what I've already checked. Got a call into their tech department (whatever..) but hoping you smart guys can tell me what I'm missing. So far... - Checked Master cylinder - has fluid in it. No leaks in lines or system that I can find. I have fluid out of the master cylinder to proportioning valve but nothing out of proportioning valve to the front brakes. It's my understanding these particular proportioning valves are not adjustable?? - Replaced proportioning valve (PV71 from the Right Stuff) - still nothing to the front brakes so thought I might have an intermittent problem with the master cylinder especially with the pedal going to the floor. - Replaced master cylinder - looks like they use one from like a '68 Chevelle or the like...disc front, drum rear application with an 1.125" bore. Bench bled it and I get fluid with like 3/4" to 1' stroke on the piston. - Checked brake rod protrusion out of power booster with pedal depressed - Almost 2". - Verified brake rod to be 1/16" from face of master cylinder piston. - Hooked up brake lines - still no fluid to the front calipers- back brakes bleed like normal but still little to no pedal. I'm at a loss as to why it lost brakes in the first place plus has the same issue as before. I'm obviously not on the right path. What the hell am I missing here?? Here's the valve fwiw..
I think your master cylinder is not bled properly. “ I get fluid at 3/4 to 1 inch stroke” fluid should come out at the first movement of the master cylinder piston. Most of the time when the brake pedal goes to the floor and you have no leaks and fluid in your master cylinder, you have a faulty master cylinder. Bones
It could be an incompatible proportioning valve. Have the application double checked. I use aftermarket ones only. Does the pedal still go to the floor now and then? That is almost always a bypassing master. Is the master going full stroke? Disconnect the line coming off the front master port and have someone work the pedal. Are the lines hooked up backwards?
Your pic of the gold valve: See the black rubber boot on the far right? Make a simple bracket to hold the pin "in" under that boot while bleeding. (GM service manuals show the GM tool to hold the pin in while bleeding, it is like a L bracket that is held by one of the valve mounting bolts) .
the reason for the pin; There is a spool type valve in there, and I would bet it is now moved over to be blocking the front circuit. pushing in on the pin lets fluid get past that blocking valve if it is over to one side .
Thanks guys. I've got the MC back off the car so will bench bleed it again to make sure we're good there. I did double check with The Right Stuff that it is the correct valve for the application but past that they were no help. F&J - I did see that pin under the boot. I'll give that a try!! Any idea why it may have moved with both valves...old one and new one?
This is the style pf PV I prefer. No moving parts except for the bias if you need it. https://www.summitracing.com/parts/sum-g3910 I can go along with the advice in posts # 6 & 8.
Its designed so that is you have a leak, and the pressure drops on the front or rear, the valve snaps to one side, and saves the fluid/and non-effected brakes. They make a tool, that you screw out the brake light switch, and screw in the pin/tool, and it holds the valve centered during the bleeding process. I'm guessing there's a loose fitting/leaky line or caliper that caused a low pressure condition that hasn't been corrected, that caused both valve to do the same thing.
Perfect and thank you again guys. I'll get back to work on it here in a few hours and will let you know how it goes. Thank you all again - I really appreciate all the help and input.
The piston in the Prop valve moved blocking the front. There is a tool that screws in the top where the light goes to hold it centered while you bleed the brakes. First you have to center the valve then put in the tool. When you are done remove the tool and put the light switch back in.
If you google GM combination valve and then click on "images" you'll get a bunch of cutaway pics that show the innards of these valves and makes it a lot easier to understand how they work.
I remember depressing the pin with a screw driver or small punch, I don't think you need a special tool. It will shuttle if you bleed the brakes, just remember to reset it after you do each circuit if you don't want to get the special thing or remove the brake switch.
When you bled the new system, that valve did it's job. The valve moves to one side IF there is no equal pressure in both circuits! The piston not only causes the brake-warning light to come on, but also blocks fluid to the "no pressure" side of a failed system. You can't just push that pin in momentarily, you need to hold it in when you are bleeding. Some other P-valves DO use a tool screwed into the brake warning switch hole to hold the valve, but "before it moved", however, the one you show, just hold the pin in, which will allow fliud to the fronts.
I certainly agree with checking that valve,,, but - With Valve swapped out and master swapped out it's still the same issue? Brake fluid is pretty easy to outsmart. It goes where it can when pushed.
If this is a case of the pin in the PV flipped, then there must have been a problem in the front brakes to cause the pedal to go to the floor in the first place.
The metering (or hold-off) valve prevents any fluid to the front discs until about 100 psi master cylinder pressure, and has nothing to do with the pressure differential switch. Pushing that "pin" bypasses the metering valve during low pressure bleeding. Additionally, the pedal should NOT go to the floor if one system goes down with a dual master. The master has to be able to full stroke (open circuits) before the pedal bottoms out.
Thank You Guys...THANK YOU!!! The pin was it and I would have never figured that out. Drove it about 30 miles last night and getting ready to leave on a 700 mile trip with this car and the roadster in the morning. After closer inspection found the p-side caliper hose was weeping so changed that out as well. Vacuum bled it and now have a great pedal. Thank you all again - I really do appreciate all the help.
I just love these threads where a problem is described, opinions are given, with explanations of why they think their answer is correct, and how the systems work. The OP tries the different solutions and THEN comes, back and tells us which solution, or combination of solutions fixed the problem, and also thanks the people who helped him. This is how the Hamb is suppose to work. No BS, just technical support.
Thank YOU for coming back with the cure. Any mention of these multipurpose valves usually starts an argument about what they do. Vacuum, low pressure bleeding and gravity bleeding are ways to avoid having that valve shift. Enjoy your ride!
The reason arguments/discussions develop about these multipurpose valves is because many still do not know what they do, or how they work, yet will throw out misinformation. That's why I always recommend keeping a custom non-stock disc/drum-disc-disc brake system simple by using only an adjustable proportioning valve and necessary residual valves. The pressure differential "valve" did not shift in this case and cause any issues. The metering valve simply needed to be bypassed, and that's what pushing the pin does, allowing low pressure bleeding. Period.
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