Disc Brakes Lock Up

[FredB]

I recently converted my 60 Apache to disk brakes and late model rear end. After driving 4 to 5 miles the disks are so tight the truck will not move until it sets for a couple of hours. The first time I drove it about 7-8 miles and the front hubs were hot and the truck would not move after I stopped. I bought a new combination valve and changed it out but that did not help. I let it cool and drove it home. When I got home it would hardly move so I cracked open one of the front bleeders and then the wheels would spin. What cuold be causing the pressure to not release? The rotors are used, the calipers and master cylinder a NAPA rebuild. The booster is a new Early Classic (I pulled the vaccum check valve also and didn't help). The front hoses do make a U that is lower than the caliper and the combination (proporting) valve is mounted up and down instead of lengthwise. Could this be my problem?
Thanks
Fred

[protrash64]

I read about this problem once before and looking at your pics I was wondering if your brake lines are too close to your exhaust. Its hard to tell but it mite be something to look into. Heat= pressure.......

[FredB]

Thanks for taking the time to help with my problem guys. After I got the truck home the first time heat is something I thought about since a couple of the lines are only an inch and a half from the headers. I let the truck idle for 20 minutes and none of the lines were hot they were barely warm. I'll try to run it around the block and check a little further. The front hoses are new and the master is supposed to be for disk. The master has two different line fitting sizes and reservoirs unlike the non-power drum/drum I took off
Fred

[Sammy]

Never seen one(PV) mounted like that before. My first impression was to check the front hoses for collasping internally. But after viewing your pic I am wondering if you have a disk M/C or you used your drum/drum M/C which could also be your problem.

[Sammy]

Mine mounted.

[joe231]

you will have to make sure it is a disc/drum master, there are dual reservoir drum/drum units that keep pressure on the wheel cylinders, believe they have a residual valves built in...which would make your front rotors drag and build up heat and possibly build more pressure...

[fastom]

Is the pushrod at the pedal adjusted correctly?

[joe231]

good thought, do you have some brake pedal free-play?

[protrash64]

I thought of this earlier too....you should have some play before the rod presses in the MS/CYL. Dont know the exact spec, if there is one.

[FredB]

Thanks for the ideas guys. At rest I have a 1/4 inch clearance between the pedal and the stop. I am starting to think the problem is with the master. From looking at masters online at NAPA mine appears to be a Disk/Drum for manual brakes (1980 JB1 option)the large reseivor is in the back opposite most power disk setups I've seen. The booster from Early Classic has a long pushrod that goes in the master and the only masters I see online with a deep hole in the piston are for manual setups. Anybody know what master they use? 71/72 maybe?
Thanks again for all of your help Happy Easter!

[protrash64]

I dont know.....but Im glad to be learning this stuff...FredB, there is a thread over in 60-66 about a ms/cyl setup by ChiefRocka....maybe it would help you out.

[62LS1]

My first thought is there is a problem with the pushrod length between the booster and the MC. Something is holding your brakes on all the time, as evidenced by you releasing the pressure by opening the bleeder screw/s. I remember that some vehicles use two holes in the brake pedal assy. One for power brakes and one for manual brakes. This could also be part of the problem.

Good luck.

[joe231]

I had the same problem with a '66 booster and a mid '80's master the '71-'72 is what I used it has the deeper recess for the longer rod coming out of the booster, works great

[Painter D]

I had the same problem with my truck even with all new stuff. It turns out it was either the wrong MC ,or the rebuilt MC was bad. Either way I swapped the MC and it fixed the problem.

[FredB]

Thanks everybody for your help. A local said to trace the problem get the brakes locked up and start at the master and work backwards to the calipers opening lines as you go until the disk is no longer locked up. If you open the line at the master and the problem is gone you know the master is not letting fluid back into it. Same with the combination valve ect. I'm going to try this tonight and pick up a 71/72 master anyway since they are cheap.
Fred

[ERASER5]

If you have a drum/drum MC or a disc /drum MC with the lines on backwards, the 10lb residual valve WILL bite you in the butt! A 1/2 ton Disc/drum MC is reversed to the 3/4 ton Disc/drum MC.

Just more info to help you trace the problem.

[Lorretta]

I just went through this exact same problem!

But it was on my 1966 Pontiac Lemans. I installed a 9" dual diaghram booster from a late 70's Corvette on my car, and a master cylinder from a 1970 Camaro. I had planned on doing 4 wheel disks but I couldn't pull it off with the bolt in axles, but that is another story.

So I converted to front disks and kept the rear drums, added a booster and a dual reservoir master cylinder and the appropriate lines.

In between the booster and the master cylinder, there is a pushrod. Mine had an o-ring on it, that was preventing the pushrod from fully seating in the booster. I removed the o-ring and the brakes dragged much less.

Then I measured the master cylinder piston hole depth, and compared it to the amount coming out of the booster. That is when I noticed the mounting depth of the master cylinder versus the mounting depth on the booster. The master cylinder housing protrudes into the booster about 1/4" longer than it should, causing the pushrod to slightly depress the piston in the master cylinder, causing the brakes to drag.

I spaced the master cylinder away from the booster, no more issue.

Hope this helps!

Good luck.

[joe231]

Ok so on a side note, on the same subject......I have a master from a '71 1/2 ton, front disc, rear drums.....can anyone tell me which port is for the front discs? front or rear?
here is a pic of the master

[cparman]

Here is a simple way to prove that there is not enough clearence between the push rod coming out of the booster and going into the master cylinder. I think this is where your problem lies as well. Unbolt the master cylinder without disconnecting the fluid lines, and put a flat washer between the booster and master. Tighten them back up and go drive your truck. Use the brakes plenty and see if either it takes longer for the problem to arise, or that the problem is gone. If this makes it go away, you need to pull the rod out of the booster and grind a tiny bit off of it. Some are threaded and adjustable. What I do when we do a disc brake install at my shop is to put a bit of clay/putty on the end of the master cylinder when I first bolt it up and tighten the bolts, then remove the master and see that the clay was not completely crushed. If it was crushed I would take a bit more off till I have a 1/16 or so not crushed. I have seen somewhere that they make a tool for adjusting this, but my method works for me. Patience, and a little fiddling in this area and you will LOVE your new disc brakes. My experience adjusting the rod under the dash is that it only adjusts the height of the pedal.

P.S. I like the way you mounted the prop valve, and your work in running the lines looks great!!!!!

[bentfab]

Originally Posted by FredB

I recently converted my 60 Apache to disk brakes and late model rear end. After driving 4 to 5 miles the disks are so tight the truck will not move until it sets for a couple of hours. The first time I drove it about 7-8 miles and the front hubs were hot and the truck would not move after I stopped. I bought a new combination valve and changed it out but that did not help. I let it cool and drove it home. When I got home it would hardly move so I cracked open one of the front bleeders and then the wheels would spin. What cuold be causing the pressure to not release? The rotors are used, the calipers and master cylinder a NAPA rebuild. The booster is a new Early Classic (I pulled the vaccum check valve also and didn't help). The front hoses do make a U that is lower than the caliper and the combination (proporting) valve is mounted up and down instead of lengthwise. Could this be my problem?
Thanks
Fred

Hey Fred,

It appears in the pic (correct me if I'm wrong) that the line coming off the top side of your combo valve which is the big nut is going to the front. Should be going to the rear and the front should be the two smaller ports which should go to the front. If they are ran proper then the master might be your problem. On the master the larger bowl is intended for front disc. Disc pads wear quicker so they need more fluid volume in the master to compensate fluid level loss as the pad wears. Also if it's plumbed back words your master might have the residual valve holding line pressure. A drum brake needs the pressure to over come the drum spring and hold the shoes close to the drum so you don't loose pedal.

[FredB]

It's FIXED! Thanks everyone for their help. The lines for the master shown were correct although I tried it both ways. Problem was the master was incorrect! The master was for a manual disc/drum setup (the large resivior was in the back). I changed to a 1971 power disc/drum master and problems was cured (had to switch the lines also). Seems around the mid 70's GM tried to make stuff idiot proof by putting a shallow push rod hole in power masters and a deep one in manual ones. I had told the guy at the parts store I needed a master with a deep hole to fit my power booster and he failed to read the description that said the one I got was for manual brakes. Oh yea the 71 master was cheaper than the one I had too!
Fred Boogaard