Master cylinder piston not returning--Semi OT

[Joe Flanagan]

I say, "Semi OT" because this has to do with my 1956 Chevrolet pickup. But the brake system is pretty much identical to our old Plymouths, so someone might benefit from this post. So there.

Anyway, I rebuilt or replaced everything on my truck's brake system. It's drums all around with a single chamber master cylinder, just like our Mopars. The brakes were working great until yesterday, when they suddenly went to the floor. There are no leaks anywhere under the vehicle. I pulled all the drums and there are no leaks at the wheel cylinders. The drums all turn freely, so it's not a case of brakes not retracting. I looked inside the master cylinder and could see the piston head through the relief port in the bottom of the reservoir. It should clear that port on its return when you let up off the brake pedal. Instead, it's stuck where it is. It will push back when you step on the pedal, but it won't return all the way.

So any ideas how to fix this? There is another hole next to the relief port, which is shown in another recent thread here on the forum. It does not go all the way through. It just looks like a bottomed-out hole. I read that there should be a tiny hole at the bottom but I can't for the life of me find one on my master cylinder. I've poked welding wire in there, a sewing needle, everything. Even looked at it before I installed it and could never see that it went all the way through. Is it supposed to?

Thanks for any advice you can offer. I participate in another forum for my Chevrolet but sometimes it's like pulling teeth trying to get an answer to my questions. Makes me miss the good old P15 forum. I should be back to the Plymouth before too long though, barring anything crazy with the pickup.

[oldodge41]

Joe there are two holes in the Mopar master cylinder resevoir. The one is indeed tiny and plugs very easily. I have run a tiny drill bit through the little one when I couldn't find the original hole. It is probably a little bigger than original but it worked just fine.

[oldodge41]

This post shows it well.

http://www430.pair.com/p15d24/mopar_forum/showthread.php?t=31696&highlight=master+cylinder+hole

[Joe Flanagan]

I saw that post. It's the countersunk hole I'm talking about. You guys are sure there's a pinhole that's supposed to go all the way through? The weird thing is, my brakes were working fine until last night and I'm positive that hole was either non-existent or plugged up. So I don't know why all of a sudden there's a problem.

I bled them again today to see if it would make a difference. First thing I noticed before I started was that the fluid in the reservoir was dark gray. Yesterday it was completely clear. This is a rebuilt master cylinder that had some pitting, which I honed. I didn't get it all out, but reduced it quite a lot. All the brake lines are new and the wheel cylinders either rebuilt or new. So I bled the brakes again today but it didn't make any difference. When I was done, the fluid in the reservoir was clear again, but I saw a little black shred of something floating in there. It looked to me like a tiny piece of rubber, which I'm thinking might be from one of the new rubber parts, which makes me think maybe the pitting in the bore is worse than I thought and is tearing up the rubber parts. Maybe there is debris lodged somewhere and the piston can't work like it's supposed to.

[oldodge41]

I know for a fact the Mopar master cylinder has a tiny hole in that countersink. I don't know if it is causing your problem. If you are seeing shreds of rubber you could have some creating a bind for the piston if it gets stuck in a pit or hole and lodges under the piston when you push the pedal. Not near the force to push it back as when you push it in.

[Joe Flanagan]

That's what I'm thinking. And I'm told this master cylinder does have the pinhole and that it is notorious for clogging up. Last thing I want to do is remove and disassemble but it's looking like I don't have a choice.

[oldodge41]

Yes, sounds like its gotta come off for a look.

[Rusty O'Toole]

There is a little pressure valve at the bottom of the master cylinder, did you change that? It is easy to overlook.

Disconnect the brake line and blow the master cylinder out with an air hose. Whatever is inside will come out like shot from a gun. So just give it a puff. Hold a rag over the end to catch the parts.

Clean out the cylinder, hone and put a kit in, it should be all right. The bit of floating rubber is bad. Something could be floating around and clog up the port from time to time.

If you already did the cylinder you should not even have to take it off, if it is the kind that goes under the floor. If it is the kind on the firewall you will have to take it off.

[Joe Flanagan]

Rusty, no I didn't see the pressure valve. I'll get a closer look at it tomorrow. I honed the cylinder and installed all new parts. Would I be shooting air into the port for the disconnected brake line?

[BobT-47P15]

Joe......if the above mentioned things don't fix it.....you probably could

have it sleeved by a shop who does such things. There are usually ads

for that in Hemmings Motor News. Just a thought. Good luck.

[Rusty O'Toole]

Yes if you shoot air in the brake line it will blow out whatever is in the cylinder.

[Joe Flanagan]

I had the master for my Plymouth re-sleeved by Apple Hydraulics. I was trying to get away without doing that for this vehicle but looks like that isn't going to happen. I know, I know. Brakes are the LAST thing you want to skimp on.

[oldodge41]

Joe, If you decide to go the new route. Hagens Hiway Auto Supply lists new ones available, as well as kits. I have dealt with these guys and liked the service. They do sleeves as well. I bought sleeved wheel cylinders for my '41 Dodge from them. They came complete with new kits installed at a good price.

http://www.hagensautoparts.com/1956/CHEVROLET+-+Truck/3100-H+1%2502F2ton+116wb%2C+3600-J+3%2502F4ton+125.25wb%2C+3800-K+1ton+137wb/BRAKE+-+Cylinders%2C+hoses%2C+kits+%26+shoes/3100-H+1%2502F2ton+116wb/

[Joe Flanagan]

Thanks for the link. I already ordered a new master cylinder from another company, but I see Hagen's offers it for about the same price.