Front brakes locked up after new master cylinder installation

[matts70]

I installed new front brake calipers (Chevy disk conversion years ago) because one was hanging up pretty bad. It fixed my problem, but they felt mushy, so I replaced the master cylinder with a new Raybestos. Immediately, the front brakes were both dragging hard. Replaced the flexible frame-to-transfer-case brake line. That didn't do anything. Disconnected the rod from the brake pedal. That didn't do anything. Removed the master cylinder, voila, no more dragging.
I guess it's a bad MC.
Anybody else experience this?

 

[Timmy390]

The break pedal rod inside the cab sets the break pedal height. You need to adjust the rod sticking out of the power booster where the MC connects to it and or the rod in the back of the booster or both.

I had to adjust mine when I changed boosters due to an internal leak. I had to adjust both ends.

You can verify this by slowly attaching the MC testing for drag as you go.

Tim

 

[Viperwolf1]

Or it has a drum brake residual pressure valve in it. You can remove the valve by pulling out the tapered seat in the front brake port. Should be a rubber plug and spring under it.

 

[matts70]

It is not an adjustable rod. Manual brakes. 1970 version.

The break pedal rod inside the cab sets the break pedal height. You need to adjust the rod sticking out of the power booster where the MC connects to it and or the rod in the back of the booster or both.

I had to adjust mine when I changed boosters due to an internal leak. I had to adjust both ends.

You can verify this by slowly attaching the MC testing for drag as you go.

Tim

 

[Timmy390]

Or it has a drum brake residual pressure valve in it. You can remove the valve by pulling out the tapered seat in the front brake port. Should be a rubber plug and spring under it.

True I forgot about that. The last 2 drum drum MC's I bough didn't have them front or rear.

Tim

 

[Broncobowsher]

So if I am reading this correctly...
Brakes drag with new master.
Remove the manual brake actuating rod, still drag.
Remove the hydraulic lines from the master, no drag.

Yes, that is a bad master cylinder that doesn't have the transfer ports correct and not releasing pressure.

 

[Rustytruck]

if you have a booster the rod between the 2 is too long. might trying spacing out the master cylinder. or adjusting the rod. the brake rod must be allowed to go all the way back to allow the pressure in the system to go out the small fluid transfer hole at the inside bottom of the master cylinder.

 

[matts70]

OK. Installed a another new MC. Same problem—front brakes locked up!
WTH??

 

[matts70]

No booster. Manual brakes.

if you have a booster the rod between the 2 is too long. might trying spacing out the master cylinder. or adjusting the rod. the brake rod must be allowed to go all the way back to allow the pressure in the system to go out the small fluid transfer hole at the inside bottom of the master cylinder.

 

[Timmy390]

Try backing off the MC mounting bolts checking to see if the pressure releases.

Did you check the new MC for RPV's? Take the brake line loose at the MC and stick a small nail or pick inside the port. If you hit rubber the valve is there. Metal it's not.

My PB setup has an adjustable rod between the brake and the booster.

Tim

 

[matts70]

The newest one from Napa maybe has pressure valves in the front and back ports—there’s something rubbery, or springy, in the holes when I insert a tiny wire into each one. Fish around a while, and the wire will find its way past it though.

Funny, when I called Raybestos tech support when I had the first one, they told me it didn’t have a pressure valve.

 

[Timmy390]

Sounds like they're in there......

My cheap one from the local store (drum/durm stock 74 MC) didn't have them in either port.

Found this. Last post has some instructions on removal
https://classicbroncos.com/forums/showthread.php?t=85818

Tim

 

[matts70]

It really looks like there isn’t a cone to remove. It looks like all one piece in the port.

 

[DirtDonk]

Now on that same subject, I've run disc brakes before with an original master with residual pressure check valves in both circuits. No problems whatsoever.
So while I think it's still possible that it could be your problem, it's not guaranteed.

But as suggested, and it's the same test we do with power brakes, loosen the fasteners holding the master to the firewall. With the pedal rod connected to the pedal, is there any forward pressure that's being released when you loosen the master? If so then the rod is too long for your setup and the pedal is coming up against it's stop (if there is one?) before the master is bottoming out on the firewall.

You can alternately check for pressure at various stages pretty easily. If loosening the line fittings at the side of the master cylinder releases the locked up brakes, then your pressure is coming from the master and is not getting locked in to the system later down the line. Such as the H-block or prop valve.

Hmm, just re-read your first post and I see that you did disconnect the pedal and it did not make any difference, but that once you removed it from the firewall the pressure was released.
If that's still what's going on, then it has to be an RPCV with too much resistance.

You can still test it by cracking the line from the front brakes at the master cylinder to see if it releases. If so, then you're half way to removing the offending check valve anyway.

Good luck.

Paul

 

[DirtDonk]

There should be a brass cone "seat" visible with the line off.
If so, you can thread a sheet-metal screw gently into it and pull to remove. You're looking for something behind it.
If it's indeed machined into the master cylinder body, then I can't see how there would be a check valve in there either.

Paul

 

[matts70]

Rear brakes port has a definite cone. The front brakes port (nearest the firewall), if there is a cone, it is tiny, no bigger than a #6 screw. I can't see any other seam indicating a cone. I did screw in a screw and pulled like hell with pliers. Nothing.

The only time front brakes released was when I disconnect the front brake line from the master cylinder. Each MC came with 2 pedal push rods--a short one and a long one. Neither is adjustable. The brake pedal can come back further, it's only being stopped by the clip holding the rod in the master cylinder.

There should be a brass cone "seat" visible with the line off.
If so, you can thread a sheet-metal screw gently into it and pull to remove. You're looking for something behind it.
If it's indeed machined into the master cylinder body, then I can't see how there would be a check valve in there either.

Paul

 

[matts70]

OK.
After poking a wire into the front brakes port on the MC several times--I kept hitting something that felt like rubber, but what?--I put the MC back on to try something else and bled it. Now the left brake is dragging a little bit (no big deal) and the right one is totally free. Beats the hell out of me.

The Chevy conversion was done about 11 years ago and it's never been great. I hoped a new MC finally would remedy this and get rid of the mushy brake feel. I had put on new calipers because one was dragging and put on new rear wheel cylinders too. It doesn't stop any better than it did before. Mushy brakes, the fronts never lock up, one rear brake locks and the other one, not-so-much.

If I did it all over again, I'd stick with drum brakes all around.

 

[1970 Palmer]

Have all (front and rear) of the rubber brake hoses been replaced? If they are old, you get get some of the same symptoms. Spongy pedal feel, and many times the brake fluid will not return easily to the MC because the return pressure is less than the foot pressure.

It's just an idea.

John

 

[DirtDonk]

Because you were not having trouble before, I think we've been ignoring the possibility of the calipers hitting the knuckle. With new pads it should be back to business as usual, but with old worn pads maybe the calipers (or one of them) is touching now.

When you look down over the tires can you see plenty of daylight between the calipers and the knuckle castings? Should be at least 1/8" but more is better.

There is no logical explanation from the master cylinder effecting only one caliper of course, because it doesn't work that way. But if one side is closer to locking up than the other, then the pressure from the master would tip the scales.
There is also no logical explanation for pressure at the master cylinder, when there is no pressure on the pedal. If the pedal can still come up, and the rod is not holding the piston inward slightly, then pressure buildup is coming from another dynamic.
Perhaps a stuck piston (yes, in both masters), or the aforementioned RPCV, or the hoses as was mentioned, or perhaps even some debris in the back of the master blocking the piston from returning all the way to neutral.
We know it's in the master because loosening the master and/or loosening the hard line does release the pressure.
There's just not a "normal" reason from most experiences.

But just to rule it out, check or re-check the clearances at the calipers. Then when you're really ambitious, pull them off and check the condition of, and lubrication on, the sliding pins.

Paul

 

[Ranchtruck]

You may have damaged the front residual check valve enough for it to leak back now when you were sticking the wire in the port? They're a delicate rubber duckbill.

I did a chevy disc conversion with stock manual master on a 71, and the pedals hard as a rock. Could use a brake booster, but it stops well if you stand on it.

Mushy brakes could just as easily be the rear drum brakes out of adjustment or the shoes not the right arc to match the radius of the drums. Ran into that with aftermarket brakes for my Tacoma. Guy at O'reilys asked if it had 10 or 11" brakes, got the ones for the 11". Put it together and the brakes were mushy, and then started dragging when the auto adjuster started taking up a click at a time when I'd use the parking brake. Took it back apart to figure out WTF was going on, noticed the shoes didn't match the radius of the drums. Went back to O'reilys and had them look up the part number on the box, does it match the 11" drums? Yup. Then asked if it matched the part number for the 10" drums? Yup. Wait, what? How is that supposed to work? Had them take a brand new drum off the shelf and match it to another set of shoes they had and same problem. Shoe touched in the middle and had an 1/8" gap on each end. They couldn't even find the right shoe in their system, that's all they carried. I took a grinder to the new shoes and removed half the life out of the middle of them to get them to fit and work properly.

The same issue occurs if the drums have been machined too much and the diameter's too big for the arc of the shoes.

The clearance between caliper and drum brake knuckle is key as well. There certainly needs to be grinding done to the knuckle when the conversion is done to make sure the caliper can move freely through it's whole slide pin range with no pads in. If it's hitting and twisting sideways, that will make the pedal feel mushy because it's twisting first before clamping force is applied.