I have removed the proportioning valve from my car - straight lines to the back "T" and a "T" straight from the front of the master cylinder. Does this change the recommended bleeding order? I had assumed that the unintuitive factory order was due to the prop. valve. Now that it's out of the way, should I just go from farthest to closest? Tom454 and Norval, what have you guys done to get the best results?

BTW - someone had suggested in a past thread that I move the master cylinder away from the booster to free up some flow - that worked like a champ! I put in some spacers as an experiment, and when I pressed the pedal and opened a bleeder, WHOOMPH - huge amounts of air came straight out. So to whoever that was, thanks a ton!

Ben

 

Why disable this safety feature of your car? Is the car for show only? How will you prevent rear brake lock-up?


As to your question, given your description as to how you have disabled the system, it would seem like the sequence would be unchanged. (although the front and rear brakes are no longer connected)

 

Mapman - thanks for your concern. This topic was actually hotly discussed a few months ago, with some very well-known guys on the forum duking it out. Tom454 actually took it upon himself to do some pretty extensive testing with and without the valve - I can't search the archives for old threads anymore, but I believe his conclusion was that the brakes would function better without the valve. Norvalwilhelm is another who has performed this mod, and he loves it. There were others who have made this change, or who have recommended it. The difference in piston size between the front and rear calipers should provide all the "proportioning" I need. And as an added plus, if I develop a leak, I still have a pedal, as opposed to before, when my leaking caliper caused my pedal to suddenly go to the floor. Independent circuits are great, and the calipers themselves have the engineering in place to keep the rears from locking up (theoretically). If it doesn't take, I'll change it back. No big deal.

EDIT - as far as your comments about bleeding order - I think that the changes I made have made this car's system more like a "normal" car, and thus I should bleed in the "normal" order. I could be wrong, though, if there are other factors that I haven't thought of, so I appreciate your thoughts, mapman. Any other comments?

Ben

 

I have a suspicion that the guys who say they lose all their brakes when just one corner goes out had bad proportioning valves in the first place. I just bled my brakes last night, and I would still get a solid pedal (though after more travel). If I was bleeding the rear, the fronts still appeared to work and vice versa, and I still have my proportioning valve.

That being said, IIRC our weird bleed order was due the proportioning valve. With your setup, it's not going to matter if you bleed front or rear first, and your probably should be going from pass to driver side.

Can you give more detail on what you did with spacing your MC from your booster? I've never heard of that one.

 

I'm sure it's been stated and discussed before, but our vettes
do NOT have a proportioning valve. The pressure proportioning
is achieved through a difference in the piston area from front to rear.

The "valve" should be more appropriately named "distribution
block with integral warning switch". It has a small piston
between the front and rear circuits. When one circuit lets go,
the other pushes on the piston and closes a warning switch.
So, all you lose is the warning light. Safety issue ? not really - IMO.

 

I really don't think that's true. As I recall up to '68, the p-valves were adjustable, and '69-> just went with fixed adjustment valves.

 

Bingo! - on my '70, all that thing does is power the switch when there is a pressure differential front to rear... However, you can purchase aftermarket proportioning valves if so desired.

 

Since the back and front are totally seperate system bleeding order doesn't matter. I still bleed the passengers side first since it is furthest from the master cylinder. I have never been worried about bleeding order and never have a problem.

As for saftey I feel since the 2 ends are completely seperate you can not loose one end and have it affect the other.
Unless you have tired it don't knock it.
With the stock system if you loose a line you are in trouble and will frantically pump the pedal trying to get the car to stop.
Without the proportioning/light switch you will hardly notice one end gone. The pedal remains firm.
I did this also to the mustang and immediately notice a big improvement.
Yes you will lock up the back but my fronts are also locked up in a panic stop but I feel the rears are doing there job by trying to slow the car down.
I have run both ways and really like the improvement removing this light switch made.

 

wcsinx - I didn't do anything exotic. Just a couple of washers, maybe 3/8" total, on the studs that mount the MC to the booster. Looking at the master cylinder with the cover off while someone else operated the brake pedal from inside the car, it was very obvious that more fluid was moving than before the spacers. I guess it has to do with the throw of the master cylinder pushrod... ? But I'm honestly a little stumped. Someone had suggested in an unrelated thread that a member might have gotten a PB MC instead of a non-PB (or visa versa), so that could be my problem too.

NHVette - I know my Haynes manual refers to the valve as a "combination valve." I don't know what years used what, but I do know that the 1975 was referred to as a "combo valve," meaning a combination proportioning valve and distribution circuit with a warning indicator built in. I'm not worried about the light - I didn't need a warning on my dash to tell me my brakes were shot!

And from what I can tell, the proportioning function of the combo valve is redundant because, like you said, the brakes are proportioned with piston size.

I think I'm gradually answering my own question on the bleed order (with a little help - thanks!). Obviously, with separate circuits I can bleed front and rear independently, but I think that I will be starting at the passenger side, and move to the driver's side.

Thanks for the input, all. If anyone else feels the need to chime in, it's welcome.

EDIT: Didn't see norval's post, so and I hope to have the same results.

Ben

 

People keep saying that, but the thing is I was still getting a solid pedal last night when I was bleeding the brakes. If the prop. valve connected the front and rear the way you're saying, the pedal would have just sunk right to the floor with any bleeder opened, correct? If your p-valve is working properly, I don't think it's supposed to do that.

I respect your experience. I really do. I'm just being the devil's advocate here. How did your brakes "improve" by removing this?

 

I think the non-PB piston had a shallow rod well and the power piston had a deep well. So that would be my guess too. With my pedal all the way up, my piston comes fully back to its stop.

 

[


I respect your experience. I really do. I'm just being the devil's advocate here. How did your brakes "improve" by removing this?[/QUOTE]

On the vet I ran MT wrinkle wall 12.50 sportsman pro tires, on the mustang I had MT ET 11.5 inch street slicks.
Before removing the valve both cars felt ok but it didn't feel ever like the back was doing it's job.
After removing the vet valve and going for a ride you wouldn't believe the difference. It felt like someone grabbed the back bumper and was hanging on. Those big stickly tires were finally trying to stop.
I did the mustang next and if you can lock up big 11.5 slicks you have serious braking.
Our cars have about 70% braking on the front and 30% on the rear. Why not make both ends 100%.
We want be sticky tires on our car why not make them work going and stopping..

 

Here it is to the best of my understanding...

When the prop valve is triggered by a pressure inequality (leak), the piston inside moves towards the leak to stop off the faulty circuit. I have assumed that the increased volume in the circuit caused by the movement of the piston, and the corresponding drop in line pressure, was responsible for the increase in pedal travel. I'd love to be corrected on this though, because at least then I'd know for sure...

Ben

 

The factory must have decided that they weren't worth bothering with either. It might have been taken out even earlier like 78, but I don't know for sure.

 

That makes perfect sense, and corresponds with my bleeding experience 100% I would still get a solid pedal with a bleed screw opened, it just took more travel.

 

Nope, the design changed in 78, but they were still used.

 

On the vet I ran MT wrinkle wall 12.50 sportsman pro tires, on the mustang I had MT ET 11.5 inch street slicks.
Before removing the valve both cars felt ok but it didn't feel ever like the back was doing it's job.
After removing the vet valve and going for a ride you wouldn't believe the difference. It felt like someone grabbed the back bumper and was hanging on. Those big stickly tires were finally trying to stop.
I did the mustang next and if you can lock up big 11.5 slicks you have serious braking.
Our cars have about 70% braking on the front and 30% on the rear. Why not make both ends 100%.
We want be sticky tires on our car why not make them work going and stopping..

Are you running those tires at all 4 corners though? I think you might be a special case. Most of us don't have that kind of traction back there. In a panic stop, which end locks up first for you?

 

I think what still has me a bit confused is the fact that I did NOT have a solid pedal. By "more travel" I mean "to the floor with minimal brake engagement"

Ben

 

Then I would suggest that you needed a new prop. valve. There was no way I could have pushed my pedal to floor even with a bleeder opened (I tried). And I look under my booster ... yep, prop. valve present and accounted for.

 

But back on topic, Ben, how do you like your brakes without the valve? Do you run the same tires at all corners? If you lock them up, which end locks first?