[Hotrodjohn71]

Do you see anything wrong with these?

Changed my (1" bore) MC and I'm working on a brake pedal that goes down about a quarter way then.have very grabby front brakes (discovered while driving in the rain). Pedal goes down then very very touchy and grabby.. one toe and almost lockup fronts.) Front shoes are new, no contamination. . 4 wheel power drums on the car.

The rod from booster to MC has about 1/8" play and brake pedal pin is in lower hole.

Im concentrating on the rear brakes lately because the stock 2 stage MC has to pressurize the rears before the fronts because of the linear design of the MC.
Im thinking the sinking pedal is rooted in problem with the rear, but I dont see anything wrong.

I built a pressurization brake bleeder which I added about 15psi (at the MC) to the rear brake circut, and with the MC unbolted from the booster , and in a level position, I flowed alot of fluid through the rear bleeders and saw no air.

After all that and for good measure, I had a helper get on the brake pedal and (several times each side) do 3 presses and a 4th press and hold where I bled them again. No air.

Also while under the car and turning the rear wheels, I noticed they would stop turning when the pedal was pressed so the rear brakes were activating, and adjusted to have a slight drag.

Emergency brake cable loose and brake shoes on the pins.

I hate to take this to a brake shop and spend a bunch of money. I thought I knew how to fix these things. This just dont make sense to me.

[graygator]

What car?
My first guess is not to suspect the rear brakes at all.
Don't take it to someone else. Just take a step back and start over.

[Hotrodjohn71]

It's a 67 Firebird, all stock. 4 wheel power drum brakes.

[Sirrotica]

Mismatching brake linings can easily make drums brakes grab. Look at the edge of the brake lining, and see what designation the front and rear shoes have, FE, and FF are common designations. It tells what composition the lining is. Ideally you would like the front and the rear to match on a 4 wheel drum brake system.

Just to show you how much it matters, I used to drive 18 wheelers, the mechanics for some reason only replaced one half of a set of brakes on the steer axle. When braking it would literally rip the wheel out of your hands. They tried all kinds of adjustments, and parts swapping to cure the problem. Finally someone looked at the brake linings side to side, and determined that they didn't match. Replace the linings with new ones, and no pull at all. They screwed around with it nearly a week before they found the problem.

You say the problem showed up after changing brake shoes, so it's not far fetched to have a mismatch, all brake linings are not created equal, and it's not hard to check.

My preference is new linings, and new shoes that are riveted, not relined shoes, and not bonded linings. For over 50 years twisting wrenches my findings are that cheap linings, and relined bonded shoes will give you inferior brake jobs.

[Murf]

Did it always do this or has something changed recently? Are you sure the booster isn’t doing something weird. Maybe try a known good one if you can find one. Just spitballin!

Good luck!
Murf

[Sport69]

Did the 67 come with a distribution block?

[Goatracer1]

First of all are you ABSOLUTELY sure that the 2 master cylinders are the same bore? If your new one has a SMALLER bore it could easily cause your brakes to be very touchy. Don't assume that they are. Measure them. You don't have to remove the new one completely. Leave the lines attached and just pull it back from the booster and measure it. Also is there play between the booster push rod and the bottom of the hole in the back of the master?

[Hotrodjohn71]

Thanks for the replies.
This car has a distribution block but not a proporting valve. I went through the distribution block when I changed the MC. It was clean as a whistle.

I checked the length of the booster to MC rod by using a little bit of dumb dumb clay on the end of the rod and bolting everything together and I adjusted the rod to have a little under 1/8" of play.

The reason I had to dive into the brakes is because the car started developing a front brake drag that got progressively worse.

At some point, I deduced that the problems cause might have been that the booster to MC adjusting rod, being an adjustable style, might have screwed-out over time causing the MC to not return to its resting position.

This makes sense because when the brakes would drag, I had a very very high pedal that responded immediately with the slightest push.

So when I was adjusting that rod length while installing the new MC, I noticed that there was no resistance in the crimp side of the rod threads and it just turned freely in the threads. So I made a locknut the same diameter of the rod itself and that took care of that.

I did check the MC bore size between the old and new. It was 1".

I just picked up a new set of new rear brake shoes of the same style and brand as the fronts. I'm going to install those soon and see if theres any improvements.

[Vroom_vroom]

Is the side with more meat on the front or back? It’s been a min but I believe the bigger pad goes on the front, if not the breaks are garbage and the e brake will not hold for anything


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[ts3189]

Primary shoe on backwards on passenger side?

[67drake]

The shoes are installed correctly on both sides.

[Goatracer1]

I would sooner think your problem is in the front. Jack up the car and have someone step on the brakes while you check to see if the front brakes release completely. You could have front brake rubber hoses that are partially restricted.

[Sport69]

Have you checked the MC? I don't care how new, or how cautiously rebuilt things are, Murphy's Law always supercedes that!

[Hotrodjohn71]

I changed out my rear brake shoes with a similar set to the front. It makes sense that after the brake drag, there could have been some glazing on the rear shoes.
So things have improved a little bit. Right now I have a pedal that goes down about 1 1/2 inches and then I have brakes. I could live with this but I would like to try to improve it all the same.

How necessary are these residual pressure valves that used to come in the outlet ports of factory drum brake master cylinders?

My research shows that these are not installed in any of the aftermarket Master cylinders including the one I bought. What was the purpose, and how necessary are they?

[Goatracer1]

They hold a small amount of pressure on the wheel cylinders so you don't have to move the pedal as far to get the pistons to apply. Especially important on older cars where the master cylinder is under the floor.

[dmorg1]

I also have yet to see an aftermarket MC with the residual valves installed.


A few years back when I spoke to MP Brakes about the absence of them in their MCs I was told that the residual valves are not absolutely necessary but they are recommended and to not use them at my own discretion.


The problem with the aftermarket MCs is that they are not machined to accommodate the factory type residual valves, at least the ones I have seen. So, one option is to add the inline residual valves.

I did not want to go with the inline valves so I took a chance, hoping there was enough material in the aftermarket casting to allow for machining. Fortunately, there was and it worked out well for me.




Dave

[Hotrodjohn71]

Dmorg1, do you have a source for the rpv's? I have an older MC that I could rebuild. But I dont see rpv's in the rebuild kits.

[dmorg1]

The last kit I used was the Raybestos MK1108 and the RPV was included but this was several years ago.

A quick vendor search is showing that currently all MC rebuild kits with the RPVs are discontinued. The Raybestos online catalog does not even recognize the MK1108 part number.

The ebay is has a current listing for the MK1108 kit, but this kit contains only one RPV as it is for a manual disk setup.


If anyone happens to have access to an old Raybestos catalog it would be most helpful in finding the part number for a kit that contained two valves.

Dave

[67drake]

The last master I rebuilt was about 10 years ago. The NAPA kit had replacement valves in the rebuild kit, BUT moot point as the kits are unavailable all together for my car at this point (‘71 GTO, manual drum brakes)

[Shiny]

It has been many years, but when I discovered the check valves were missing from my new MC, I chose to add an aftermarket RPV. Without one, I could imagine the rear brakes being slow to engage but I don't think the lack of residual pressure in the rear would cause the front brakes to suddenly become grabby.

I think you are getting good advice to double-check the MC bore diameters and pushrod setup.