Check valve in 64 master cylinder?

[James B.]

Question: I believe I have heard that the 64 with drum brakes requires a check valve in the brake master cylinder. Reason being that the drum brake shoes had lots of travel and the check valve kept some pressure in the lines so the shoes would not fully retract when the brakes were released; that kept the shoes in close proximity to the drums.

Background: I always wanted a 65 Fuelie but couldn't afford one so about 12 years ago I bought a 64. I was frustrated with the drum brakes so over a couple of years bought disc brake suspension and converted the car to discs about 5 years ago (the drum brake suspension is rebuilt and in plastic bags in my basement so when it comes time to sell the car I can offer to convert back). I didn't want to cut the original brake lines so kept the car as a single circuit system with the master cylinder in use with the drums. I have been warned that using a master cylinder with the check valve will keep pads in contact with the discs and overheat the brakes. I have never noticed a problem but have driven the car very little and only on short drives. I have sourced a correct rebuilt 64 master cylinder but am hesitant to use it for the above reason. Also, the bore is 7/8" and I believe the 65 was 1" (power brakes). If that is the case I will have to find a 1" bore single circuit cylinder w/o check valve and save the 64 unit for when I put drums back on the car. Will appreciate help and advice, Regards Jim B

[Timothy B.]

Re: Check valve in 64 master cylinder?

James,

What master is on the car now. If you want a 1" bore master why not get a repro part or service part so not to much $$$ and use it until or if you ever decide to convert back. No check valve with disc brakes and it's easy to remove.

If the car brakes work ok with the 7/8" bore master just keep using it but remove the check valve.

[Stuart F.]

Re: Check valve in 64 master cylinder?

This check valve business is new to me. I'd had a 64 master on my 63 for a number of years until recently and both look functionally the same. In fact the 64 unit is the common replacement for the 63 unit due to the latters scarcity. I believe the mechanical self adjusting feature on the 63/64 drum brakes does a good job at keeping the shoes close to the drums - if it is working right. When I changed my brakes over from sintered metallic to metrix ceramic a few years back, I changed out all of the adjusting hardware and applied all the service manual techniques to ensure they were preadjusted correctly. I then followed the instructions to "road set" them with a number of reverse stops. I continue to be mindful of the need for the reverse stops to help keep my brakes adjusted properly by doing this in clear parking lots once in a while, and not just by backing out of my driveway. So far I'm very pleased with there performance and I don't have to worry about leaking piston seals of disc brakes or dragging pads due to collapsing brake lines causing a "check valve" effect. I use Dot 4 fluid, and change it out (flush the system) at my local brake shop every other year or so primarily due to the concern for high humidity water absorbtion here in Florida.

Where is this check valve you refered to anyway?

Stu Fox

[John H.]

Re: Check valve in 64 master cylinder?

This check valve business is new to me.
Where is this check valve you refered to anyway?

Stu Fox

Stu -

ALL drum brake master cylinders have had an RPV (Residual Pressure Valve) since the 30's - it's in the output end of the bore (figure 3 on page 5-2 of the 1964 Corvette Shop Manual, far left end of the bore) and maintains 10 psi in the fluid system; without it, it would take almost a full pedal stroke to get the shoes out to the drums when braking. DISC brake systems don't use them, as disc pads are in light constant contact with the rotor surface by design.

[John H.]

Re: Check valve in 64 master cylinder?

Question: I believe I have heard that the 64 with drum brakes requires a check valve in the brake master cylinder. Reason being that the drum brake shoes had lots of travel and the check valve kept some pressure in the lines so the shoes would not fully retract when the brakes were released; that kept the shoes in close proximity to the drums.

Background: I always wanted a 65 Fuelie but couldn't afford one so about 12 years ago I bought a 64. I was frustrated with the drum brakes so over a couple of years bought disc brake suspension and converted the car to discs about 5 years ago (the drum brake suspension is rebuilt and in plastic bags in my basement so when it comes time to sell the car I can offer to convert back). I didn't want to cut the original brake lines so kept the car as a single circuit system with the master cylinder in use with the drums. I have been warned that using a master cylinder with the check valve will keep pads in contact with the discs and overheat the brakes. I have never noticed a problem but have driven the car very little and only on short drives. I have sourced a correct rebuilt 64 master cylinder but am hesitant to use it for the above reason. Also, the bore is 7/8" and I believe the 65 was 1" (power brakes). If that is the case I will have to find a 1" bore single circuit cylinder w/o check valve and save the 64 unit for when I put drums back on the car. Will appreciate help and advice, Regards Jim B

Jim -

That's correct - you do NOT want to use a '63 or '64 master cylinder with your full disc brake system; the internal RPV (Residual Pressure Valve) in a drum brake master cylinder maintains 10 psi in the hydraulic system, which will eventually glaze (or fry) your disc brake pads, which are already held in light constant contact with the rotors by springs behind the pistons. An easy fix is to replace the existing master cylinder with a '65-'66 Corvette manual disc brake master; it has the correct bore size and fluid displacement capacity that matches your system, and doesn't have an RPV.

[Stuart F.]

Re: Check valve in 64 master cylinder?

John;

Thanks for the clarification. I believe the RPV doesn't function as a true "check valve" as otherwise there would be no way to bleed off the pressure in the lines. Perhaps it is more like a two way valve that performs both pressure and relief - like in a late 60's gas cap.

Stu Fox

[Dick W.]

Re: Check valve in 64 master cylinder?

John;

Thanks for the clarification. I believe the RPV doesn't function as a true "check valve" as otherwise there would be no way to bleed off the pressure in the lines. Perhaps it is more like a two way valve that performs both pressure and relief - like in a late 60's gas cap.

Stu Fox

It is a backflow check valve Stuart. Allows full flow out and holds 10 p.s.i. in the lines on return

[James B.]

Re: Check valve in 64 master cylinder?

John, very helpful and basically what I suspected. I believe someone added power brakes to my car (while it had drum brakes). The master cylinder is not original and has a 1" bore ( casting number 5464343 so not a corvette part & I don't know what it's from). So I guess I can rebuilt it and remove the RPV. If the bore is pitted I will source a 65 manual brake master cylinder. But a question, I believe PB 65 cars used a dual circuit 1" cylinder. Would you suspect I would have any trouble using the '65 manual 1" bore cylinder with my PB booster? Further, out of academic interest, why did Corvette in 65 use a single circuit on manual brake cars and dual on PB cars? Thanks and Regards, Jim B

[John H.]

Re: Check valve in 64 master cylinder?

John, very helpful and basically what I suspected. I believe someone added power brakes to my car (while it had drum brakes). The master cylinder is not original and has a 1" bore ( casting number 5464343 so not a corvette part & I don't know what it's from). So I guess I can rebuilt it and remove the RPV. If the bore is pitted I will source a 65 manual brake master cylinder. But a question, I believe PB 65 cars used a dual circuit 1" cylinder. Would you suspect I would have any trouble using the '65 manual 1" bore cylinder with my PB booster? Further, out of academic interest, why did Corvette in 65 use a single circuit on manual brake cars and dual on PB cars? Thanks and Regards, Jim B

Jim -

The '65 manual disc brake master cylinder should work fine if the pushrod hole in the rear piston is the same depth as the one in your current master cylinder. I suspect the dual-circuit system starting on '65 power brake Corvettes was an opportunity for a low-volume "field trial" of the system that was to become standard in VERY high volume on '67 full-size cars. Trivia - My boss at the time at Chevrolet Production Engineering "forgot" to sign the paperwork (ECR) releasing the dual-circuit disc brake distribution block for the '67 Impala/Caprice, which caused a real panic at Delco-Moraine and bungled the launch of that part (which restricted the volume of J52 brakes for Impala/Caprice for the first two months); stuff happens.

[John H.]

Re: Check valve in 64 master cylinder?

John;

Thanks for the clarification. I believe the RPV doesn't function as a true "check valve" as otherwise there would be no way to bleed off the pressure in the lines. Perhaps it is more like a two way valve that performs both pressure and relief - like in a late 60's gas cap.

Stu Fox

Stu -

As Dick noted, the RPV is a full one-way check valve, that holds pressure at 10 psi. Starting with passenger cars with disc/drum systems in 1967, the RPV was rubber, spring-loaded in place behind the brass seat at the master cylinder's drum system outlet - much smaller (and simpler/more reliable) than the multi-piece valve used previously in drum/drum systems. Photo below shows a typical disc/drum master cylinder of the era - note the RPV at the rear (drum) outlet, and no RPV at the front (disc) outlet.Power Front Disc Rear Drum Master Cylinder[1].jpg

[Bill C.]

Re: Check valve in 64 master cylinder?

It has always been my understanding that the "residual pressure valve" was necessary in drum brake systems for this reason, due to the fact that the brake shoe return springs would pull the shoes back against the pins and in turn push back the wheel cylinder pistons and the cup seals behind them, pushing the fluid back and the system slightly pressurized and keeping the cup seals expanded and sealed.

These cup seals have no spring expanders in them. (I think later, new seals were developed with expanders in later cars)

Depending on how the shoes were adjusted or how worn they were there could be quite a bit of travel of the piston/seals in the cylinders, and a corresponding longer travel of the master cylinder piston stroke.

There was just the one cylinder, the master cylinder that would be returning "out" of the hydraulic system, pulling fluid behind. If the master cylinder returned quickly with a long stroke and for some reason any wheel cylinders pistons/seals that were not being forced back just as quickly do to weak brake shoe return springs or some other type of drag, a vacuum could potentially be created in the hydraulic system.

If this happened air could potentially be pulled into the system do to the cup seals not being held expanded by fluid pressure (as there were no built in expanders), and easily allowing air to be pulled into the hydraulic system. That condition could not happen at the master cylinder end as its the fluid reservoir on the other side of its cup seals and so would only potentially pull in more fluid, but not air.

So the residual pressure valves were actually one way restrictors of volume ensuring that a minimum amount of pressure was kept in the hydraulic system against the wheel cylinder cup seals, keeping them expanded and sealing fluid in and air out as the return springs pushing the shoes/pins back against the fluid kept pressure against the fluid regardless of how fast the master cylinder piston returned potentially creating a vacuum in the hydraulic system.

The residual valves used in different masters are rated at different pressures and different designs based on system size/design but all doing the same job.

The inability to use such a valve in our disc brake systems using caliper piston cup (lip) seals was because it would have held slight pressure in the system holding the pistons/pads against the rotor with excessive force.

So many suffered at the hands of inexperienced brake work from air being sucked right on by the lip seals if the rotors were not running true do to oscillation of the pistons in and out of their bores, creating a vacuum , and , well you know the rest.

[James B.]

Re: Check valve in 64 master cylinder?

John, very helpful and great insite as usual. After "stuff happened" did you become your boss's boss?

As an aside to all viewing, since most reading this thread are purists I am sure they will cringe at disc brakes on a '64 (honestly it hurts me a little too). I will only offer that the car is a past NCRS top flight L84 that is well carred for. Someone added PB & PS, and I have collected original parts to change all back to original specs when I go back to drum brakes. I accept that I am the current caretaker and want to someday pass the car on as original as possible.
What I appreciate is that on posting this thread I expected the first response to be condemnation for screwing up an original car. Obviously that didn't happen and I very much appreciate the constructive and helpful advice. Thanks to all and Regards, Jim B

[Timothy B.]

Re: Check valve in 64 master cylinder?

James,

Power steering and power brakes are two nice upgrades but I certainly don't see anything wrong with drum brakes. I installed P/S on my 67 and I love it..

I don't understand why some people change out there drum brakes to disc but the reason may be to use a dual circuit master. It's a nice safety item but a well maintained drum brake system should be fine.

[Stuart F.]

Re: Check valve in 64 master cylinder?

I agree with Tim on the drum brakes. I like mine, specially when I have just replaced mine once and yet helped my son do caliper changeouts on 4 C-3's already. Back in the day, when my sintered metallics were fresh and hot, I was in a Corvette cruise in the western suburbs of Chicago. It was one of those parades where you get a "whip lash" effect of cars speeding up and then rapidly coming to a stop. Several times I had to really climb on them and all my junk in the back came flying forward, and the C-4 behind me had to lock up and slide around me to keep from hitting me. My brother, who was with me, was very impressed. He said he'd never felt a car stop so fast without lockup. He was a little chubby too and said the seat belt dug into his belly, Ha!

Sorry I was a little confused in my earlier posts about the check valve thing. I'm nearing 74 now and a lot of things I think I did yesterday was really some years ago. I've had my 63 L-76 since new and she and I have lived through a lot these past 50 years.

Stu Fox

[Joe R.]

Re: Check valve in 64 master cylinder?

Stu,

I was a little confused also as most check valves or limit valves are external of the Master Cylinder. John was a little confused also bringing in the 65 and lumping it in the disc brake arena. There were many 65's with drum brakes. It was an option to delete the disc brakes and stay with the drums.

JR