Proportioning Brake Valve Problem - Kindly Help

[Paul Benton]

Having had a lot of problems bleeding the brakes on my 1970 C3 after removing and replacing the master cylinder during some restoration work I am having difficulty in getting any brake fluid to flow from the back caliper bleeders. Master cylinder was bench bleed before re-fitting. I have both vacuum pump and pressure bleeder. Brakes working fine before this task was undertaken

I am convinced the proportioning valve (its the brass one as attached) has been un-centered in this process, so any tips for how to re-center it would be appreciated.

Also have seen contrary tips when bleeding of 1) front raising front wheels and 2) raising back wheels of the floor, whats recommended?

Kind regards
Paul

 

[curious bystander]

Try stamping hard on the brake peddle several times to re-centralise the valve.

 

[Paul Benton]

There is no resistance in the brake pedal, it goes straight to the floor, as I can t get fluid to flow to the back calliper bleeders hence still full of air.

 

[johng]

Paul, I've also read your post on the US forum. It sounds like you haven't tried bleeding the front brakes yet. If the proportioning valve is blocking off the line to the rear brakes then the line to the front brakes will be open, so you should be able to bleed them. If you can then chances are your master cylinder is still okay and you just need to centre the valve. How long did you try gravity bleeding for? When I rebuilt my brakes I left it gravity bleeding for several hours on each caliper, it was a very slow process. Eventually I got a bit impatient and switched over to my vacuum pump, this seemed to continually expel air bubbles along with the fluid, but I'm not sure if the air was just being sucked in via the nipple threads. It did give a fairly solid pedal though. Finally I had a go with the traditional pressing the brake pedal method which firmed up the pedal properly.

 

[johng]

Just had a further thought! If you bleed the front brakes using the gravity or vacuum method and then press the brake pedal to see if it's firm then the proportioning valve is going to close off the rear line (if it hasn't already done so). If you can get fluid to flow out of the front brakes, then it might be worth having another go at the rear brakes before pressing the brake pedal, especially if you didn't give the gravity method very long on the rears. In fact with the gravity method you might as well do front and rear together anyway.

 

[Oneball]

Easy way to check if the valve is centred is to measure the continuity across the switch.

 

[Paul Benton]

This is of great help, thanks for the guidance.

Paul

 

[Paul Benton]

One ball; what is the method to test the continuity place a meter probe on valve contact and the other to earth?

 

[Paul Benton]

Problem solved; based on the guidance given here an on the Corvette forum to eliminate it I took the master cylinder apart to find it had a corroded barrel and the seals were scored by this.
But the discussions were very helpful, and I have much a deeper understanding of the whole braking system now and how to trouble shoot it.
So new master cylinder and whilst I am ordering, a new proportioning valve to eliminate any issues with it seemed sensible.

Many thanks for all your help here.

Kind regards
Paul

 

[stealthyflatfish]

Paul , just to throw my three penny worth in as I went through a similar scenario, and spent many an hour researching and head scratching.
I know you've found the problem now with the master cylinder. ( good )
Although it's called a proportioning it's not really, but I suppose they had to call it something and as it sits and looks like a proportioning valve, that's what it gets called.
It's just a brass block with a switch inside it, the switch part can be removed by unscrewing it and can be cleaned and tested. The brass block that's left has nothing else in it, it's just a junction box for the four brake pipes The purpose of the switch is to purely illuminate the brake light on the dash in case of fluid loss. If fluid is lost the switch will make and illuminate the dash light.
The master cylinder on a Corvette is dual circuit ie two reservoirse so if you loose fluid out of one circuit, you still have the other circuit with which to apply the brakes, all be it only the front or back brakes will work. The proportioning part of the brake pressures are actually controlled by the master cylinder as the outlet connections are different sizes, and therefore apply different pressures to front and rear brakes.
With the car sitting on the floor on level ground and you standing on the driver's side you might notice the master cylinder leans back slightly towards the passinger compartment, you can check this with a spirit level sat on the master cylinder, this traps air at the top left hand side of the MC and makes bleeding almost impossible, so jack the back of the car up until the MC sits just slightly down at the front, ( you can use the spirit level again ) this will stop the air getting trapped at the front. Once you have bled the brakes the peddle might still feel soft. With the top of the MC still off, cover the top of the MC with something like clear cling film to seal the top. Get someone to observe and keep pumping the peddle slowly to expel any remaining air out of the system, should be good to go.
I hope this all makes sense and makes it an easier job.

 

[Paul Benton]

Thanks for that, another learning for what seemed to be a simple task, but due to design is a real headache, but thanks to the corvette community sharing lots of information and tips I have pieced together now by consensus the best procedures.
Who would have thought that just filling a system with fluid would have so many issues.!!