daytonablue64

I just completed converting my 64 over to disk brakes (salvaged off a 73) and the brakes now have excessive drag.
I'm currently using the original drum brake master cylinder and I'm wondering if there is a residual valve within it that may be limiting the pressure bleed off at the calipers? If so is it easily removed?
Can anyone provide any insight on this problem?
The calipers were rebuilt before the install so there was a lot of air to bleed off. I can tell there is still a bit of air that still needs to be bled off, could air in the lines cause excessive brake drag?

I have the dual circuit m/c that I'll be installing at a later date but that will take some re-plumbing of the lines and I'm in a hurry to get it out of a buddy's garage and get it home where I can re-plumb at my leisure.

Thanks

CabSav

I'm not sure about M/C compatability, but maybe you could first try checking to see if your drag is mechanical or hydrauic but undoing a bleed nipple with some clear hose on it so that you can see how much fluid, if any, squirts out under pressure. Fluid should only dribble out if at all. If you get a squirt, you probably have a m/c problem, if none you have a piston/caliper problem.
Good luck,
Peter

67pete

Master cylinder and distribution switch need to be compatable with having disc brakes installed. '67 up utilizes dual chambered master cylinder.....

MikeM

The residual valve is located in the very front of the master cylinder bore, on the front end of the piston spring. It's usually a cup shaped washer with little holes in it and a rubber flapper. Just take the valve out and put the guts back in.

I've run disc brakes before with that residual valve still in place and it didn't keep me from driving the car FWIW.

67L36Driver

In addition to the above:
Master cylinder pushrod may need to be adjusted a tad shorter.

wmf62

i agree, the residual valve should be removed. i ran my disc conversion for 30 years and had no problem, BUT the brakes dragged to the point that the pads squealed and the wheel was hard to rotate.

no harm done, but we removed the valve from the M/C and now the car can be rolled with the slightest push.
Bill

midyearvette

did you change the rear brakes too??...if so, stainless steel brakes makes a m/c that works on 4 wheel systems with no junction block...fyi

toddalin

I run front disks/rear drums on my '64. When I first did the conversion I left the '64 M/C and it worked sort of OK, but there was always some excessive drag on the fronts. You should also know that the volume of fluid held in the '64 M/C is "minimal" for disk use.

I replaced the '64 M/C with a single reservoir M/C from a '65 and things got better. I later changed that out to a dual cylinder M/C from a '70s vintage and it all works fine. But I think the '65 single M/C had a better pedal feel.

BTW, if still have the '65 M/C and lid, both in excellent shape if you want to try to go that route. Just let me know.

KC John

I agree that the residual valve is what is causing the drag. You can drive the car home as it is, but you should remove the valve as soon as possible if you are going to drive it for any length of time.

The valve is used to keep the brake shoes up against the drum so that they don't have to move a large distance to engage the brakes. The disk brakes don't need that because they are always in slight contact with the rotor.

Tintin

Actually, you need a junction block and a residual valve with disk/drum brakes because the brakes will retract when the fluid returns to the master when you release the pedal and there will always be a weird feel to the brake pedal going forward. You didn't indicate if you only did the fronts or all four, in any case I think you would be wise to convert to the 67 master as it is quite a lot safer and has the appropriate bore for disk brakes. As your car is not original, it will make no difference to the NCRS crowd.. here is a guy that can probably help you with your problem and sells the 67 master as well:

http://ecihotrodbrakes.com/

If you are going to drive the car any distance, having a lot of brake drag is not good because:

A) You will get crappy mileage and the car will be slow

B) Yor discs can warp from the heat of being under constant high friction

C) You will wear out your pads pretty quickly


I converted my 64 to discs and I can spin the wheels with one hand and they rotate with only bearing friction...

JohnZ

Actually, he doesn't want a '67 master cylinder, as it has no RPV for the rear (drum) system on his car, which will give him a long pedal and minimal rear brakes, especially at low speeds.

Disc/drum conversions work best with an engineered system - use a dual master from a disc/drum Camaro/Nova/Chevelle; it has the correct bore/displacement for the front discs, and an RPV behind the rear outlet seat for the rear drums.

Photo below shows a dual drum/drum master cylinder - item 1 is the RPV spring, item 2 is the rubber RPV, and item 3 is the pressed-in brass tube seat. Disc/disc cylinders don't use RPV's, and disc/drum cylinders only have an RPV in the outlet for the rear system.

toddalin

JohnZ, Tintin:

If you are refering to my disk/drum setup, I did use a M/C from a '70s vintage Corvette, but also plumbed the 10# residual valve and proportioning valve into the rear lines. The proportioning valve is under the dash and I can adjust it as I drive. It shows as the red handle under the dash.


I also put a "hill holder" (line lock) on the front disks. It replaced the front distribution block and you can see it right next to the steering box adjustment.


BTW, if you retain rear drums, you've GOT to have a rear residual valve one way or another or you're in for some wild times. Initially, I tried it without and I would often have to pump the brakes once or twice to get the rear wheel cylinders in position. That's not a lot of fun, especially in "panic stop" situations.

I dont' think the original poster ever noted if it was a disk/drum setup, only that he was using a stock '64 M/C with the setup and was wondering if that was causing the drag.

wmf62

right or wrong, and with no problems, i used, and still use, the C1 M/C without the residual valve and without a proportioning valve. residual valve in the M/C and no proportioning valve with disc/drum brakes for 30 years, and then disc/drum with no M/C residual valve and no proportioning valve for about a year, and now 4-wheel discs, again with the original C1 style M/C and no residual valve and no proportioning valve
Bill

Tintin

The link I posted for ECI sells 67 masters and seperate residual valves, I have both and it works magnifico. The other masters work too , but as I intend to go 4 wheel disc at some point this made the most sense for me...

daytonablue64

Thanks for all the great feed back, you've confirmed my suspicions that the drum m/c is the likely culprit and that it can be easily alleviated by removal of the internal RPV.
To clarify things, I now have disks on all 4 corners.
To get the car home will be about a 50 mile ride at highway speeds so I don't want to take chance with overheating the new rotors on that long of a drive.
I plan to pull the m/c tomorrow, remove the RPV, bleed the brakes again and keep my fingers crossed that takes care of it.

Thanks guys

daytonablue64

Since I'll be upgrading to the 73 dual circuit m/c after I get it home again...it would be great to see some examples of how others have re-plumbed from a single m/c to a dual m/c.
It looks like I could re-use the original 4 way brass junction block to continue to feed the front brakes, by merely plugging the old port that used to feed the rear drums.
I could then plumb in a short new line from the new m/c's rear circuit port to a convenient location where I could use a 2 port brass block to splice the new line to the original line heading back to the rear disks.
Has anyone else done it this way?

toddalin

You see my set-up above.

Even if I ran front and rear disks, I would still plumb in a proportioning valve to the rears to get the best possible brake balance. I would just leave out the external residual valve.

IIRC, GM actually did this on L88s from the factory (but I could be wrong).

JohnZ

That's correct - the J-56 HD Brake package was part of the L-88 option, and it used a Kelsey-Hayes adjustable rear proportioning valve mounted below the master cylinder to set rear proportioning to best suit the track conditions encountered. Several photos of the installation in Noland's book, and in the J56 section of the Assembly Manual.