wcsinx - I have no doubt at all that my prop valve was not working properly. It was stuck - no question. However, I couldn't get it "un-stuck!" Tom454 did some tests with a perfectly functioning (freshly hand-rebuilt) prop valve, and had similar results to my own, as did norvalwilhelm. So we all want to know your secret!

As far as how it works now - dunno yet! The car is still in the air... I only had a few minutes to mess with it the other day, and likely won't get another chance until this weekend. I'll keep ya'll posted for sure.

One thing I do already know - for the first time since I have owned the car, the rear brakes are grabbing hard, even with lots of air still in the system. My current test (to avoid raising and lowering, wheels on and off again) is to jam a pry bar between the wheel studs (with lug nuts on!), and try to turn the wheel with the car in neutral and the brakes applied. Can't spin the rears by hand with the brakes applied anymore!

EDIT: I will run BFG T/A's 235 front, 255 rear, so that should help a good bit with any rear lock-up fears.

Ben

 

I have no idea. I know it's functioning (and not gutted or bypassed or anything like that) because the light came on when I was bleeding. My pedal would absolutely not bottom out with any single bleeder opened. I'm no NitroC5, but my legs ain't exactly weak either with all the running & bicycling I do.

Yeah, definitely let us know. 235/255 isn't the extreme application that Norval has, so I'm curious to know what your results are. In truth, Norval, you're proportioning the rear by putting so much more rubber back there!

I don't think you should ever be able to do that. I definitely know mine are grabbing.

 

I don't think you should ever be able to do that. I definitely know mine are grabbing.[/QUOTE]

Yeah, mine were pretty nasty. My car spend a lot of time sitting before I bought it, I think

And, yeah, Norval has a wicked setup. My car wouldn't know what to do with 305's, but the larger tires in the rear should help even out when the brakes lock up if it proves to be a problem.

Ben

 

NHVette is completely correct in his explaining the function. It is not a proportioning valve It is a distribution block which senses pressure differential to turn on the brake warning light.

For those that have replaced it, I can understand the positive aspects of increasing fluid flow in this area, the 'distribution block' passages are restrictive and full flow would decrease 'from pedal applied to lock' time.

Craig

 

Then explain why mine works like a valve. I will let anyone who so desires inspect my car. Open a bleed screw, and try to push the pedal to the floor. If what you and NHVette are saying, it should sink right down no problem. But mine does not. They really are valves.

explain

 

It sounds like your VALVE is working properly - blocking off the open circuit.

Guys, don't forget that different years have different blocks in that position. I do not know the breakdown, but I am fairly sure that the chrome bumpers DO NOT have a prop valve. But my 1975 DOES. wcsinx, your car is a '76 - I'm fairly sure that '74-77 had the same "combination" prop valve/dist block.

I wish I could point you folks to the archives... there has been some phenomenal discussion on this topic, with a few conclusions that I consider to be inarguable.

Ben

 

true dat, Zip lists one for 67-68 (which is actually adjustable) another for '69, 70-73, 74-77, and 78-82. So I wonder if perhaps in some years it was essentially a passthrough which did nothing but operate the idiot light? Maybe? So in that case, what would keep the rears from locking too easily? The pistons are the same size for all years IIRC.

 

I believe that rear lockup would be prevented by the variation in piston size between the front and rear. The front calipers have larger pistons.

Ben

 

At least in 1980 Corvettes had a Proportioning Valve.

The 1980 Corvette GM Shop Manual states in Section 5-2 Operation of the Combination Valve;

" The proportioning section of the combination valve proportions outlet pressure to the rear brakes after a predetermined rear input pressure has been reached. This is done to prevent rear wheel lock-up on cars with light wheel loads.

The valve is designed to have a "by-pass" feature which assures full system pressure to the rear brakes in the event of a front brake system failure, full front pressure is retained in the event of rear failure.

The pressure differential warning switch is designed to constantly compare front and rear brake pressure from the master cylinder and energize the warning light ......"

Sounds like several different configurations over the years.

 

I did some reading and 74 and later C3's do have a proportioning valve that sends more pressure to the front system under heavy pedal pressure. I don't understand the need for this because of the built in differential in pressure due to relative piston size. Looks like the General decided (or the government ((more likely)) a proportioning valve was required even if it wasn't needed. When you think about it, 1974 is the peak of when Government safety and fuel requirements really took hold. Based on others experience from removing their post '73 'valves', it seems that the original design of the braking system provided for correct front/rear proportioning.

I'll have to look through the archives to get more info but I can tell you that prior to 1974, proportioning valves were not used in C3 Corvette brake systems.

Craig

Edited "Added C3 - I have no idea about C1 and C2 brakes"

 

I think you got it. I looked at my Chilton's and Hayne's (yeah yeah I know) and both of them seem to indicate that there was a time when it was just a switch and a time when it was a combination of a switch and valve. Though they disagree on the year, the sentiment holds. And the statement they both made was that it was designed to limit pressure to the rear under "heavy braking" to prevent rear lockup.

I'm thinking right about now, I'd love to have the rears lockup.

OK, so what's the drill to bypass this farker? What parts do I need? I'd love to eliminate one more potential point of failure.

 

wcsinx - to bypass it you just need some line and fittings. I wound up running a straight line from the rear MC reservoir and coupling it with the existing rear line at the frame. I then ran a straight line from the front MC reservoir into a "T" to meet the front lines. You should have enough slack in the two front lines to bend them around to meet a "T."

Vettenut72 - I think you've got it. There's no question that the chrome bumper C3's didn't have the valve, but the later ones sure did. Sounds like we agree on the year that the change took place too.

Ben

 

I've never really worried about which wheel to start on. I usually suck out the old and continually add new synthetic until I'm back to clear clean fluid.

With my sticky Z-rated 255 - 275 17 inch front tires on 9 inch wheels. My rear tires always blow clouds of smoke if I stomp on the brakes. Years ago when I had 8.5 by 15 inch on all four corners the only Z-rated I could find in 15 inch were BFG T/A 245/50 series and because they were only 25 inches tall the fronts would lock. I almost bit the dust a few times when the fronts locked and I lost steering control.

 

See that's another thing that confuses me. Why did the General work so hard to keep the rears from locking? I guess ideally you'd want all 4 to lock at the same time, but if anything it's better to lock up the rear first as you won't lose steering that way.

 

I'm an old motorcycle road racer. I learned years ago that tire diameter has alot to do with how effective a given disk brake is.

Any rotor/caliper combo has a given sweep area usually referred to in sq. inches. It's actually the width of the pad being able to touch the rotor surface for one revolution.

Idealy you would want more sq. inches on the front than the rear because of weight transfer during braking. Our case on vettes is same rotor dia and the adjustment of amount of fluid to each end to work as a bias.

Tire diameter has a big effect because less revolutions per mile takes away from how effective a disk brake is. So a simple change from 24.7 inch tall Z-rated 245/50/15 to 27 inch tall Z-rated 255/50/17 inch front tires. From front tires locking all the time to my rear 315/35/17's locking up every time.

I even run higher friction front pads than rears on my 79 to also work as a front to rear brake bias. I use carbon metalic fronts and ceramic metalic rears. So my front rotors are blue/black front glowing with heat and the rears are more sintered brown. Ceramic rear pads last 2-3 times as long as my carbon fronts and so do the rotors. I v'e never counted the number of front rotors that I have replaced and I think that It's just my third set of rears I just put on this spring.

 

Gkull - do you have good luck bleeding with a vacuum pump? I've heard people say that pressure from the MC is better...

Ben

 

Absolutely not! The saying is, "locked axles lead"...if you lock up the rears first you will quickly become a passenger in an uncontrollable spin. This is simple to prove, just take a model car and place it on a table, then tip the table to get it roll. After that, lock up the rear wheels somehow and try it again...the rear will come right around.

 

All I have is a vacuum pump. Somebody on this forum. Probably Norval, made is own pressure system out of an extra MC cap. they posted it here on the forum.

All it was is to take a MC cap and drill a hole in it between the two hold down clamps. then they brazed on an air hose fitting. So you just fill the MC resevoirs clamp on your cap. Attach your air hose with it regulated down to 15 - 20 psi and just crack each wheel caliper bleed.

You just have to pop off the air hose to vent the pressure off and continually check to ensure you still have enough fluid.

 

Not my car. Although I try to always do 95% of my braking in a straight line. I do agree that if you were turning just the slightest amount your going to loop your car.

It's been my experience on the street during panic stops or on the track doing 140 mph coming into a 40 mph turn. That when I really stomp on the brakes you can hear a sliding type sound and when you glace in the rear view mirror I see clouds of blue smoke and long black wide lines.

Now when I had front locking problems you see the clouds of blue smoke coming out of the front fender wells and you realize that the car won't turn into the corner and your just going straight. So you have tell yourself to get off the brakes and try for the turn. It's amazing how the front tires catch a grip again and your on through the turn. Where if you kept the brake on you would have been biting the guard rails.


I have never looped my car do to stomping on the brakes even in wet conditions because I always replace my tires when I notice that traction is going away. I have looped my car many times because of the gas petal going out of turns

 

I did that George. I did exactly as you said and just turn the shop regulator down and clip on the air line. It is simple to bleed one back and one front, refill and do the other side.
It only takes a few minutes per wheel to bleed this way,'
I have a very high end vacuum pump for bleeding brakes and I find it useless. Bubbles contiue to rise in the tube because air is drawn in around the bleeder screws.
To me pressure is the only way to bleed brakes.

As for locking I think my front and back lock at the same time.
I believe in organic pads too to save the expensive rotors but by the 4th panic stop the organic pads are fading fast. For the first few stops the organic match the metallic but after 4 100mph panic stops the organic pads are done for and the metallic/performance friction were actually getting stronger with the heat input but they ate my rotors.