C3 1970 Brake proportioning Valve Problems
Thread Starter
Racer
Joined: Nov 2012
Posts: 367
Likes: 16
From: London
C3 1970 Brake proportioning Valve Problems
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 calliper bleeders. Master cylinder was well bench bleed before re-fitting until no bubbles and beyond (done this a couple of times). I initially tried gravity bleeding, have both a vacuum pump I have applied from the bleeder end and a pressure bleeder applied from the master cylinder end. Brakes working fine before this task was undertaken and valve photo is attached for reference.
There is zero resistance when brake pedal is pushed down, so can't use the "pump and then push pedal hard" to centre the valve and when I do this the level of fluid does not fall, however I have only so far tried bleeding from the rear callipers, I am convinced the proportioning valve (its the brass one as shown in attachment) has been un-centered during this process, so any tips for how to re-center it would be appreciated.
Also have seen contrary tips when bleeding of 1) Raising front wheels and 2) Raising back wheels of the floor, which one is recommended?
Kind regards
Paul
There is zero resistance when brake pedal is pushed down, so can't use the "pump and then push pedal hard" to centre the valve and when I do this the level of fluid does not fall, however I have only so far tried bleeding from the rear callipers, I am convinced the proportioning valve (its the brass one as shown in attachment) has been un-centered during this process, so any tips for how to re-center it would be appreciated.
Also have seen contrary tips when bleeding of 1) Raising front wheels and 2) Raising back wheels of the floor, which one is recommended?
Kind regards
Paul
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
With all bleeder screws closed, usually a quick jab to the brake pedal will re-center the Prop Valve. There is also a tool that will hold that piston centered for $12.
It could very well be off center if it detected a leak. That's what it suppose to do. Proportion the fluid elsewhere.
When a situation comes up like this, I like to read between the lines.
Quote "brakes working fine before" . . . .
So then we back track. What has changed? The master was changed. "It was properly bench bled". Or was it?
When you had the MC in your bench vise, how far did you push the MC piston in? If you went more than 1 3/8" you likely damaged a piston seal. Perhaps both seals but most likely the rear. This also voids the warranty on the new MC. If either one of those piston seals (ring) gets damaged, it will never hold pressure, nor will it force fluid to the rear bleeders such as your case.
To see photos of all these topics, click on my profile. Then photo albums, bleeding brakes.
It could very well be off center if it detected a leak. That's what it suppose to do. Proportion the fluid elsewhere.
When a situation comes up like this, I like to read between the lines.
Quote "brakes working fine before" . . . .
So then we back track. What has changed? The master was changed. "It was properly bench bled". Or was it?
When you had the MC in your bench vise, how far did you push the MC piston in? If you went more than 1 3/8" you likely damaged a piston seal. Perhaps both seals but most likely the rear. This also voids the warranty on the new MC. If either one of those piston seals (ring) gets damaged, it will never hold pressure, nor will it force fluid to the rear bleeders such as your case.
To see photos of all these topics, click on my profile. Then photo albums, bleeding brakes.
Last edited by HeadsU.P.; Oct 20, 2020 at 04:34 PM.
Melting Slicks
Joined: Mar 2010
Posts: 2,270
Likes: 375
From: Randolph nj
when they gave me my replacement i had to use the old one
think about it the only thing you changed was the master,, and your brakes worked before
I had to stop my brisket video to look this up ,,, lol
think about it the only thing you changed was the master,, and your brakes worked before
I had to stop my brisket video to look this up ,,, lol
Last edited by forman; Oct 20, 2020 at 08:02 PM.
Le Mans Master
Joined: Aug 2006
Posts: 7,427
Likes: 1,560
From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
2019 C3 of Year Finalist (appearance mods)
Google haw to adjust the master cylinder pushrod to the booster.
Last edited by Peterbuilt; Oct 20, 2020 at 09:17 PM.
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Thread Starter
Racer
Joined: Nov 2012
Posts: 367
Likes: 16
From: London
This is a power brake set-up on a 1970 C3, is a spacer needed, as I don't remember one being there when I dismantled it (although that was 18 months ago), and see attachment where one is not shown.
Kind regards
Paul
Kind regards
Paul
Last edited by PBCloud; Oct 21, 2020 at 03:04 AM.
Thread Starter
Racer
Joined: Nov 2012
Posts: 367
Likes: 16
From: London
Thanks Heads-Up for you comment;
Oh dear, the guide I followed did not warn me about that one, when I bench bled it I did push the plunger in short bursts but finished by pushing it all the way in all the way in, so as you suggested I may have damaged the seals; a new lesson learnt!
There is no leakage of fluid (just removed M/C to take a look, completely dry with no leakage along the shaft), so probably got away with it on this occasion. But as its brakes and safety is all, so are the seals easily replaced or does this mean considering a new M/C?
Paul
Oh dear, the guide I followed did not warn me about that one, when I bench bled it I did push the plunger in short bursts but finished by pushing it all the way in all the way in, so as you suggested I may have damaged the seals; a new lesson learnt!
There is no leakage of fluid (just removed M/C to take a look, completely dry with no leakage along the shaft), so probably got away with it on this occasion. But as its brakes and safety is all, so are the seals easily replaced or does this mean considering a new M/C?
Paul
Last edited by PBCloud; Oct 21, 2020 at 03:28 AM.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
When my rebuilt MC was delivered, it came in a white box. A warning label that was adhered on the box was light yellow. I almost missed the warning about the 1 3/8" max stroke. I need bright red labels to get my attention.
A quick test to see if the MC can and will hold pressure is:
Purchase two ready made steel brake lines about a foot long, with the fitting on one end that are the correct size for the MC ports. The front & rear ports are not only different diameter but different TPI also. At the other end of of your test line, simply bend to pinch it off. Manipulate the lines onto the MC ports by bending slightly.
Now, you should be able to stand on the brake pedal. And I mean stand on it without budging all day long.
If your pedal sinks, look no farther. You have an issue.
So, for a bout $20 you have two master cyl test lines that you can use over & over in the future. If you are clever you can fabricate the lines for "MC bench-bleeding kit" also.
A quick test to see if the MC can and will hold pressure is:
Purchase two ready made steel brake lines about a foot long, with the fitting on one end that are the correct size for the MC ports. The front & rear ports are not only different diameter but different TPI also. At the other end of of your test line, simply bend to pinch it off. Manipulate the lines onto the MC ports by bending slightly.
Now, you should be able to stand on the brake pedal. And I mean stand on it without budging all day long.
If your pedal sinks, look no farther. You have an issue.
So, for a bout $20 you have two master cyl test lines that you can use over & over in the future. If you are clever you can fabricate the lines for "MC bench-bleeding kit" also.
Last edited by HeadsU.P.; Oct 21, 2020 at 09:32 AM.
Thread Starter
Racer
Joined: Nov 2012
Posts: 367
Likes: 16
From: London
Problem solved; based on the guidance given and 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.
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
But the discussions were very helpful, and I have much a deeper understanding of the whole braking system now and how to trouble shoot.
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
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
The Prop Valves seldom go bad. Just one moving part. Not much to it.
However, if you really want to swap it out, make sure you order the exact yr need. They changed the design on the warning switch over the years. If you get the wrong one, the wiring to the warning switch will not work w/o some mods.
However, if you really want to swap it out, make sure you order the exact yr need. They changed the design on the warning switch over the years. If you get the wrong one, the wiring to the warning switch will not work w/o some mods.
Thread Starter
Racer
Joined: Nov 2012
Posts: 367
Likes: 16
From: London
So I guess after 50 years service it deserves a rest, and whilst it would have been nice to have kept the originals; can't take a chance with brakes and with some really good replacements out there with all the correct stamps etc. I can live with that..
Thanks to the corvette community for sharing lots of information and tips, as a result 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.!!
Thread Starter
Racer
Joined: Nov 2012
Posts: 367
Likes: 16
From: London
Just to finish this off, received my new MC and proportioning valve and fitted them, but still had some difficulty in getting free flow of fluid through the lines, this was easily resolved by reverse pressure using a pressure brake bleeder connected to the bleeder at the wheel pushing the fluid up the lines, and the whole job then became quite easy. So lesson learnt re: MC, needs to be in good condition and when bench bleeding don't push the piston in more than 1 3/8" or you will damage the seals (there was a warning on the box the new MC came in explaining that if you do you invalidate their warranty, so pretty important!!).
Also noticed what might be a "tell tale" in that when I bench bled the damaged/corroded MC lots of micro bubbles observed, whilst the new one only produced "large" bubbles so my guess is this might be an indication for an an air leak from the seals.
Thanks for all your help and suggestions.
Kind regards
Paul
Also noticed what might be a "tell tale" in that when I bench bled the damaged/corroded MC lots of micro bubbles observed, whilst the new one only produced "large" bubbles so my guess is this might be an indication for an an air leak from the seals.
Thanks for all your help and suggestions.
Kind regards
Paul
Intermediate
Joined: Mar 2024
Posts: 27
Likes: 3
Hi Paul,
I know this is a very old thread, but writing in the hope you might see it.
I have battled with my brakes for months and pretty much changed every part of the brake system, including the master cylinder.
My brake pedal sinks all the way to the floor as soon as I start the car and the booster kicks in. I still have brakes, but only when the pedal almost touches the floor.
I may not have followed the 3/8" when I bled the master cylinder (I can't remember to be honest).
Does this sound like the symptoms you had? I will buy a brake line and block it off, as someone suggested, because I am running out of ideas. I assume the proportioning
valve would not cause this type of issue?
Cheers, Matt
I know this is a very old thread, but writing in the hope you might see it.
I have battled with my brakes for months and pretty much changed every part of the brake system, including the master cylinder.
My brake pedal sinks all the way to the floor as soon as I start the car and the booster kicks in. I still have brakes, but only when the pedal almost touches the floor.
I may not have followed the 3/8" when I bled the master cylinder (I can't remember to be honest).
Does this sound like the symptoms you had? I will buy a brake line and block it off, as someone suggested, because I am running out of ideas. I assume the proportioning
valve would not cause this type of issue?
Cheers, Matt