Brake pressure regulator valve causing problems? Solved!!!!

[aeromech]

Guys,
I'm beating my head against the wall with this one. It's a 1971 bus (all stock). When the owner brought me the bus I just drove it from the street to my garage and noticed that the brakes seemed like they weren't working right. The owner didn't say a thing about brakes. All he wanted was an oil change.

So I did an oil change and at the same time a drive axle rebuild because he had torn boots. When that was done I checked his reservoir and found it empty so I filled it and bled the brakes. During bleeding I notice low flow through the rear brake lines using my Vacula suction bleeder. Once done I took it for a drive and noticed that the brakes felt strange. They would stop the bus but it really didn't feel like I was getting more than just front brake action.

Today I changed the rear brake hoses and the master cylinder. The booster looked good and had no brake fluid inside. I also changed all the vacuum hose to the booster using Gates 27231 hose. When I bled the rears I could not get good flow using the vacula. I had my wife pump the pedal and when she did that I could see flow. Usually the Vacula will suck the fluid right through the lines without any problem.

So at this point I'm left with suspecting the proportioning valve. I actually have a spare in stock or I could bypass the valve and see what happens just as a test.

Has anyone experienced issues with a bad proportioning valve? These things have been installed for 40+ years so there must be some failing.

PS- I never did find a leak so I have no idea why the reservoir was empty. I at first thought that the MC was leaking into the booster.
_________________
Lead Mechanic: San Diego Air and Space Museum
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Boeing 727,737-200-300-400,757,767
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DC9/MD80
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Fokker F28/F100
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VW Type 2 1971 (3 ea.) 1978, 1969
VW Jetta
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Capable of leaping tall buildings in a single bound

[fusername]

tap it w/ a hammer when she hits the brakes? it is just a ball and a spring afaik, maybe its sticking a bit? There is a inspection process listed in the bently manual.
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[aeromech]

There is a inspection process listed in the bently manual.

An inspection process? Well, now I'll have to break out the book and see what it says. I know it's a simple device but even a simple device can go to hell after this many years. Thanks for the heads up.
_________________
Lead Mechanic: San Diego Air and Space Museum
Licensed Airframe and Powerplant Mechanic
Licensed Pilot (Single engine Land)
Boeing 727,737-200-300-400,757,767
Airbus A319,320,321
DC9/MD80
BAe146
Fokker F28/F100
VW type 1 1962,63,65,69,72
VW Type 2 1971 (3 ea.) 1978, 1969
VW Jetta
VW Passat
Capable of leaping tall buildings in a single bound

[skills@eurocarsplus]

how about rear hoses? do they look old? i have also had 1 piston in each rear wheel sieze up, causing a odd brake feel.

i would be surprised if a prop. valve went bad. they seem to be bulletproof
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[aeromech]

[germansupplyscott]

or just bypass it and see what difference it makes - if any. you could use a flex hose about 12" long to connect the two hard lines into the regulating valve.
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SL

[skills@eurocarsplus]

[aeromech]

[aeromech]

[skills@eurocarsplus]

might be worth having a look, just to see if there is a piston stuck in the bore.
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[aeromech]

[Desertbusman]

You mean the pressure regulating valve instead of a proportioning valve. A major difference in what they do and how they do it and how they are made.
But pull the drums and see what's up first. It might be the wheel cylinders.
A bad functioning regulating valve can either cause too high of pressure to the rears and cause skidding or too low of pressure. And they can't be repaired. But they can be taken apart and cleaned out as they can get clogged up with goop and junk to the point that they won't function. There are a couple piston seals that if damaged would make it toast but chances are it just needs cleaning out.
Take it apart except probably not the ball valve seal. I used a big syrenge and kept flushing it with alcohol until it was all clean and it's working great since. Prior to cleaning it was locked tight with crap.
Bentley tells how to test it but Haynes has a fantastic cross-section diagram to show what it's all about.
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[SGKent]

inside the valve is a large ball bearing. It basically moves to cover one port to a restricted flow when the car slows hard or the nose dives on hard braking. They are very simple inside, unlike an American system. I would be more inclined to suspect rust and debris inside the rear cylinders or bleeding nipple than the proportioning valve. Pull a rubber hose off or the line coming into the tee and see if you get a good flow from that that spot with your brake bleeder.
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[aeromech]

I thought about trying to flush the line from the proportioning valve to the wheel cylinders. It very well could be the wheel cylinders but I know I flushed the system 15 months ago so there's been new fluid in there since then. It just seems like I'm seeing the same restricted flow from both wheel cylinders. The common thread is the hard line and the proportioning valve. I'm betting on the valve.
_________________
Lead Mechanic: San Diego Air and Space Museum
Licensed Airframe and Powerplant Mechanic
Licensed Pilot (Single engine Land)
Boeing 727,737-200-300-400,757,767
Airbus A319,320,321
DC9/MD80
BAe146
Fokker F28/F100
VW type 1 1962,63,65,69,72
VW Type 2 1971 (3 ea.) 1978, 1969
VW Jetta
VW Passat
Capable of leaping tall buildings in a single bound

[old DKP driver]

The Rears should just start to touch the drums before you have the fronts applied
So if you have a helper it should be easy to push the pedal slowly and check
the drag as the pedal is pushed
without engine running..


I agree the rears need Help
_________________
V.W.owner since 1967

[aeromech]

Update Tuesday Morning:

Last night I bypassed the proportioning valve using a rear brake hose. I bled again but still feel like I'm getting low flow. Then I drove it. I get the same result. When applying the brakes the pedal goes down about 1/2 way and I feel a solid stop. The bus is still rolling. I push through the stop and the bus comes to a stop. The braking action isn't as good as it should be. I don't know what this solid feeling bump or stop is in the pedal. I've had the booster off and it's dry inside and looks like it's been replaced within the last several years. The MC was replaced. The rear hoses replaced. Right now I've pulled the rear drums and the wheel cylinders look dry but I still need to pull them and see what kind of shape they are in. I'll also add the braking doesn't pull to one side or the other. I do also have a spare freshly rebuilt booster I could try if someone thinks that could be the problem.
_________________
Lead Mechanic: San Diego Air and Space Museum
Licensed Airframe and Powerplant Mechanic
Licensed Pilot (Single engine Land)
Boeing 727,737-200-300-400,757,767
Airbus A319,320,321
DC9/MD80
BAe146
Fokker F28/F100
VW type 1 1962,63,65,69,72
VW Type 2 1971 (3 ea.) 1978, 1969
VW Jetta
VW Passat
Capable of leaping tall buildings in a single bound

[busdaddy]

Did it do the same thing before the MC was replaced?
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[aeromech]

[aeromech]

Here's a run down of what I've done so far.

1) Filled reservoir and bled system. The rears didn't bleed well (low flow)
2) Took it for a ride. The bus stops but when pushing down on the pedal I get about 1/2 way and feel a stop. I keep pushing almost to the floor and the bus stops.
3) I pulled the booster and inspected it. It was dry inside and looked like it had been replaced a few years ago. So I called it good.
4) Replaced the MC
5) Replaced the rear hoses
6) Rebled and still had low flow. Drove again = no help
7) Bypassed the proportioning valve, rebled, drove again = no help
8 )Pulled the rear drums to inspect wheel cylinders. They don't look too old and don't have visual leaks but I haven't removed them yet.
9) Replaced booster vacuum line and tested check valve.
_________________
Lead Mechanic: San Diego Air and Space Museum
Licensed Airframe and Powerplant Mechanic
Licensed Pilot (Single engine Land)
Boeing 727,737-200-300-400,757,767
Airbus A319,320,321
DC9/MD80
BAe146
Fokker F28/F100
VW type 1 1962,63,65,69,72
VW Type 2 1971 (3 ea.) 1978, 1969
VW Jetta
VW Passat
Capable of leaping tall buildings in a single bound

[kreemoweet]

Your symptoms point to a failed circuit in the M/C. Too bad, being
new and all. The "low flow" from the rear bleeders may not be
related. The fluid has to travel thru a residual pressure valve, as well
as the pressure regulator, and it has some tiny orifices and parts
that could be gunked up or rusted.

I sure wonder about the missing fluid from the reservoir. Is the owner
really that ignorant?
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