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I'm bleeding my brake system for the first time after installing all new lines & calipers. I have rear disc brakes and an adjustable proportioning valve for the rear calipers. How do I get all the air out of the rear proportioning valve? Should the valve be fully open (minimum pressure) or fully closed (maximum pressure)?
Here's a pic of the valve:
You want max pressure to the rear wheels when bleeding, then adjust as needed to prevent lockup. Max pressure to the rear means minimum "proportioning". I assume you get that by backing off on the adjuster screw.
NOTE: something I learned the hard way, in case you haven't. Go get the proportioning set in a large empty parking lot IMMEDIATELY. My setup actually uses stock parts throughout (disc front), but the proportioning was way off. Those aftermarket combo valves are trash. Anyway, normal driving was just fine and you'd never know. Then one day I'm in traffic, someone tries to side-swipe me, I slam on the brakes and nearly spin out.
I added a prop valve - same as yours - to the frame rail. Good thing is it only takes a few runs to get it set well, and it's fun. But the car can be wildly unpredictable in panic situations until you get it set up.
Joe:
I pressure bled with the valve turned all the way in (CW) the valve has an arrow printed on the **** that says "INCREASE". But what got me thinking is the extra volume inside the valve when the **** is turned out (CCW) to max decrease. Shouldn't that "extra" volume when the **** is turned out need to be bled of any air? I don't fully understand how the prop valve accomplishes it's job. Is there a chamber inside the valve the absorbs the unwanted pressure? Thanks!
NOTE: something I learned the hard way, in case you haven't. Go get the proportioning set in a large empty parking lot IMMEDIATELY. My setup actually uses stock parts throughout (disc front), but the proportioning was way off. Those aftermarket combo valves are trash. Anyway, normal driving was just fine and you'd never know. Then one day I'm in traffic, someone tries to side-swipe me, I slam on the brakes and nearly spin out.
I added a prop valve - same as yours - to the frame rail. Good thing is it only takes a few runs to get it set well, and it's fun. But the car can be wildly unpredictable in panic situations until you get it set up.
I have a question regarding nomenclature and the type of valve you are using on this system.
If my understanding is correct, and anyone please correct me. The device I am viewing and reading about here relates to a proportioning valve (employed on the rear disc brakes only) and is not, in fact, a combination valve (which would otherwise be used in both the front and rear brake lines system). Is this correct or do I have a misunderstanding?
Norm:
I have modified the brakes on this car: I have C5 Corvette calipers w/13" rotors on the front and Blazer calipers w/11.5" rotors on the rear. My brake system has a combination valve from a '70 model disc-drum Cutlass that does not have a proportioning valve built-in (the later '71-up disc-drum cars do have a the valve built-in). The 1970 disc-drum cars had a separate prop valve mounted in the back of the car and the front "valve" was just a mixing valve for the brake light switch. The black proportioning valve in my pic was purchased from Inline Tube along with the '70 combo valve, they helped me put this together when I bought the brake line set from them. The valve in my pic will be used to reduce the pressure to the rear disc to prevent premature lock-up in a panic stop.
Oddball:
I plan to do exactly as you said before driving the car anywhere. i will probably do some panic stops in an empty parking lot, when it's raining just to make sure I've covered the most dangerous conditions.
So, do you guys think it was OK to bled the brakes with the prop valve cranked all the way in (CW, max pressure) or should I re-bleed with the valve turned out (CCW, min pressure)?
Rodney
Last edited by cdrod; January 22nd, 2020 at 09:23 AM.
I bet you're good. If you've got good pedal feel then you almost certainly got it all done. My assumption is "max pressure" would provide the least interference, therefore highest likelihood of getting all the air out. It's a nice sounding theory. And since you pressure bled, the current of the brake fluid also helps getting the air out of any crevices. I've never taken one of those valves apart so I don't know how they actually work.
I'm kind of surprised you're running the combo valve. Note that you could've used any drum/drum combo valve. Those just have the front splitting and the warning light stuff. Once I switch to my Z51 brakes (real soon now......) I'll just re-plumb everything and skip that block of trash. I'll lost the warning light, but meh, that light would probably give me a split second warning before hitting a tree anyway.....
Norm: You're right. My setup is all stock (except a D100 master), so I use a stock combo valve, which includes a proportioning valve inside of it. The combo also has a "metering" function, which actually slightly delays the front discs so the rear drums have a head start, as well as splitting the front left and front right lines and providing the warning light function.
What I was saying is the proportioning hard-set within my aftermarket combo valve was completely wrong, so I added a separate proportioning valve just like Rodney's. Most of the aftermarket prop valves are the same thing, just with different stickers.
Oddball & Rodney - Thanks for expanding on usage of the proportioning valve - in particular in a custom developed system such as each of you are installing.
Oddball:
What is your real name? I hate addressing people by their screen names, it seems so impersonal-lol. If I understood the guy an Inline Tube correctly, the 1970 combo valve was just for the switch (which I'm not using anyway because of aftermarket instrument gauges). I may have misspoken in my previous post, the combo valve from Inline Tube is a PR107 which was also used in drum-drum applications as well as disc-drum so I think that means there is no delay to the rear circuit as there is in the '71-up disc-drum valve. Perhaps the metering/delay was built into the rear proportioning valve for the 1970 cars?
I decided to use a stock style combo valve to simplify the install using the pre-bent lines from Iinline Tube. It seemed to me, I'd rather spend my time on tasks other than bending brake lines.
The 1970 disc-drum cars had a separate prop valve mounted in the back of the car and the front "valve" was just a mixing valve for the brake light switch.
Actually, they had a separate "delay" valve mounted under the master cylinder along with a metering valve mounted to the frame below the master cylinder.
Joe:
I pressure bled with the valve turned all the way in (CW) the valve has an arrow printed on the **** that says "INCREASE". But what got me thinking is the extra volume inside the valve when the **** is turned out (CCW) to max decrease. Shouldn't that "extra" volume when the **** is turned out need to be bled of any air? I don't fully understand how the prop valve accomplishes it's job. Is there a chamber inside the valve the absorbs the unwanted pressure? Thanks!
Rodney
There is no "extra volume". The screw is adjusting spring pressure on the valve, but that spring is on the "dry" side of the valve. You want the valve to be at the lowest possible spring setting to ensure that the low pressure used for bleeding opens the valve enough to flow brake fluid to the rear wheels.
Actually, they had a separate "delay" valve mounted under the master cylinder along with a metering valve mounted to the frame below the master cylinder.
^^^This. It was not a "mixing" valve, it was a metering valve. Every GM combo valve from 1971-up for disc/drum systesm has the metering valve function built into the front part of the combo valve. The purpose is to delay application of the front brakes for a small fraction of a second until the slop gets taken out of the rear drum brakes.
The prop valve function is built into the back of the combo valve, and the differential pressure switch function is in the middle.
That's why it's called a (wait for it...) COMBINATION valve.
Every GM combo valve from 1971-up for disc/drum systesm has the metering valve function built into the front part of the combo valve.
So this has me wondering if the 70 disc/drum valve is the same as a 4 wheel drum valve? With the difference being the drum/drum setup didn't use the separate delay valve? Or was a different metering valve used on the drum/drum setups?
So this has me wondering if the 70 disc/drum valve is the same as a 4 wheel drum valve? With the difference being the drum/drum setup didn't use the separate delay valve? Or was a different metering valve used on the drum/drum setups?
One more time: the drum/drum cars did not use a "valve". They used a distribution block that only has a piston that operates the differential pressure switch to turn on the BRAKE light if you lose pressure in one side of the brake system. There is no metering valve on drum/drum cars because the brake linkage "take up" time is the same for both the front and back brakes. If your question is about the distribution block, yes all 1967-70 factory disc brake cars used the same distribution block as was used on drum brake cars. Again, all it has inside it is the differential pressure switch function. And yes, all 67-70 factory disc brake cars used the external metering valve in the front brake circuit. The B/C/E-body cars also used separate in-line proportioning valves in the line to the rear brakes. The A-body cars did not. Interestingly, the Toronados with four wheel drum brakes also used a proportioning valve in the line to the rear brakes, due to the heavy front weight bias as compared to a RWD car. These are the only Oldsmobiles with four wheel drums to use a prop valve.
If your question is about the distribution block, yes all 1967-70 factory disc brake cars used the same distribution block as was used on drum brake cars.
Yes, that was my question. Can you clarify if the distribution block was the same as the ones used on the 71-up drum brake cars also?
According to the Inline Tube website, the PR-107 distribution block was used on 1970 disc-drum cars (with separate metering valve for the front) and 1970-'72 drum-drum cars. Here's a link to the Inline Tube website:
Yes, that was my question. Can you clarify if the distribution block was the same as the ones used on the 71-up drum brake cars also?
Yes, all dual M/C four wheel drum cars use essentially the same distro block. There are some detail differences in the bracket and such, but functionality is the same. The inside is just a spool valve that is pushed to one side or the other if there is a pressure differential, and this causes the pin in the switch contact to make a ground connection, turning the light on. By the way, the springs are really there to ensure electrical contact between the spool valve and the distro block body, since the o-rings will insulate it from the bore and prevent the differential pressure switch terminal from grounding.
Thanks for the information to everyone who posted. Everyday it seems this place is like going to Oldsmobile school with all the information that is presented.
Joe , I'm looking for the Distribution Block for my 73 oldsmoible Omega, which has 4 wheel drum brakes. Can you recommend where to purchase one, I have been looking but cannot find one.
It should be the same generic distro block used on every A-body with drum brakes, or also on Novas. Inline, Right Stuff, and just about every Chevy repro parts place has them. There might be minor differences in the mounting bracket.
January 21st, 2020 | 07:18 PM
