It's spring time again and as usual I have my typical brake nightmares.

Take the '75 out for a ride and the brake pedal is nearing the floor. Remove the wheels and find the left rear caliper decided to wizz all over the rather new brake pads. It's been about 5 years so I pull both rear calipers off and return them to Autozone for my free replacements...I see the new ones are stainless sleeved now so hopefully they'll last forever like my front set.

I bleed the system with the mity vac, get a little bit of pressure and after a few more pumps the pedal is back to the floor. Gravity bleed same thing. Drag out the 2 stage vacuum pump and still no good.

I suspect the master cylinder is original so I picked up a rebuilt one today and of course that does no good.

I finally notice some brake fluid on the floor, look up and see it running along the frame. Follow the path up to the proportioning valve. The rear body of the valve is wet! I removed the rear brake line and unscrewed the rear brass portion of the valve. Once out I see there's an O-ring and I'm hoping that's all it is, however there was a little rubber disc that popped out of the side of the valve. Under that rubber disc is an orifice. If I put my thumb over one end of the housing and put compressed air through the other side the air escapes through that orifice.

Does anyone know what that hole is for and why it's covered with a flimsy rubber disc which would never stand up to any type of brake system pressure? From the way the valve was positioned I can't say for sure whether is was leaking from the body due to a bad O-ring or if it was leaking out that hole. I hoping it's some type of air bleed that seals up when hydraulic pressure is applied?

Nobody seems to carry those old valves but the parts guy at the local Chevy dealer thinks he found it and it's still available. Just not looking forward to breaking off those top lines if I don't have to.

George

 

George
I have a 75 also. It has been a long time since mine did the same thing. Here's what I remember. At first I thought the line to the valve was leaking. Then I discovered it was leaking through the area that yours is. I think this area is designed for over pressure in the system. I think it fails more of age. Here's where to go. They have and ad on page 60 in May 2005 Vette magazine. Let us know how you do.
Steve
Master Power Brakes
888-351-8783

 

www.mpbrakes.com

They will have what you need.
Good stuff!

 

Here's another source.... apparently a new one is not available?

http://www.corvette-paragon.com/cata...8-65A41206E230

The C3 valves that I have taken apart & examined don't fit any of the categories at the mpbrakes web site... maybe they have more than what they have listed online.

 

The "valve" you are referring to is really just a switch to indicate failure of either your front or rear brake systems. If the pressure is different on either side it lights up your brake light. (If you take it apart you will see that it is simply a spool that has a cam in the middle to activate the brake light.)

Some early Camaros had porportioning valves, but no C3 have ever had them to my knowlege. The porprotioning is accomplished with the different sized (smaller rear) brake pistons.

 

My Haynes manual says 75 & later vettes use a "regulating valve which is essentially a pressure proportioning valve"

 

The valve you have was made by Weatherhead (brass body). The disc you are referring to was a "vent" that would blow out if one of the prop valve o'rings started to leak. The valve you have sounds like it has the proportioning section in it. As was suggested, yours should just be a differential switch. If the disc blew out, you will suck air into the system as well as leak fluid. I was the Kelsey Hayes proportining valve engineer for 25+ years so I can't help you with repair parts. If it's just an o'ring that went bad, it can easily be replaced. I'd be glad to take a look at it for you if you can do without it a short time. I still have access to KH parts, which probably have simular o'ring sizes.

 

I also have a 75 and after a few problems I removed this valve altogether and plumbed the back brakes to the back of the master cylinder and the front to the front of the master cylinder. It made a big difference in braking and I found if you have a problem at one end the brake pedal still feels normal. I also did this to 2 mustangs and both experience better braking and easier bleeding.
I would get rid of it.

 

Wow that's awesome .

 

Hey guys,

Thanks for all the great info and links on that valve!

Stopped by the local Chevy dealer today and double checked the part number with the exploded view. It appears Chevy really does still stock them? Part number is 339780 which matches the one in the link provided above.

Supposed to be here tomorrow morning. Cost was reasonable too. List was $80 and the company I work for has an account there so I paid under that and it dropped to $59 bucks.
Just keeping my fingers crossed that it actually shows up!

 

Cool!

 

those valves are evil. get rid of it.

 

So Duke,
Can you explain why these valves seem to make things worse when there is a failure?

Like what Norval says?

Also why the cahnge from just a switch in 69 to a different kind in later years?

 

So Duke,
Can you explain why these valves seem to make things worse when there is a failure?

When one end fails like a ruptured brake line the switch flips causing the pedal to go to the floor. Without the switch if a line fails the pedal still feel fine because one end is totally unaffected by the lose of the other end and other then reduced brakeing/depending on the end that fails, the pedal doesn't go to the floor and feels firm.
If you are bleeding brakes without this valve you never have the pedal going all the way to the floor. You never have that panic feeling when something happens and you suddenly have no brakes and start pumping like carzy.
Other then a warning light it serves no purpose and hinders if you really have a failure.

 

norval
1. built in obsolesence
2. what better excuse to buy a new car? no brakes
3. increased revenues for dealers(frustrated owners go to dealers to bleed)
4. prove to washington that safety devices don't work.
5. more revenues for body shop(smashes)

 

"The regulating valve, used on 1975 and later models....improves front to rear wheel braking characteristics during heavy breaking, when weight is transferred from the rear to the front wheels."

Haynes Manual

 

I found braking performance increased when I remove this valve and the rear end squatted down more and you could feel the back end doing it's part.
I still do not run a proportioning valve but have since gone to 14 inch front rotors. I can panic stop from 100 mph without locking the back.

 

I had the same experience and same results on a '79 Chevy 4 X 4.

 

Just a follow up on my proportioning valve:

As promised the local Chevy dealer had it the very next morning. Fit perfect and looks 99% identical to the original. Brand new, no core return needed and only $60 bucks, can't beat that.

After using the mity vac to bleed the system about 10000 times (why does a C3 have to be so damn difficult!) I got a decent pedal. Took an out of town trip today and racked up 500 miles and she stopped great each time yet I feel the pedal travel is still a bit too far.

After looking around the net I found MOTIVE PRODUCTS POWER BLEEDERS. The price of the one I'm looking at seems very reasonable and it would include the adapters to fit all my cars.

http://www.apexperformance.net/cartg...p?pid=48&cid=8

Has anyone used one of these on a C3? Any good? Looks like it would make a miserable job easy plus I'd probably be more apt to flush the brake systems on all my cars every few years like I'm supposed to.

 

Please post the part number of the part you got from Chevy.