I just finished a manual disc brake conversion on my 68 convert. I took it for a test drive and found that after a short drive, the brakes were dragging. They drag to the point that they start to smoke. The pedal is extremly firm and high which at first I thought was great. I used the combination valve off the donor car, (a 71 lemans), bought new calipers, brakes, rotors, and master. I had trouble finding a master cylinder because the hole in the back of it was shallow as opposed to the the original drum/drum cylinder that was deep. after not being able to locate a deep disc/drum manual cylinder, I cut the rod off and adjusted it so it just touched the back of the piston on the master. Can anyone help?

Ok, so now that I can't leave this alone, I went out and made sure my calipers and rotors were the same as the originals. The rotors are 1" and the calipers measure the same as the old ones. I pumped the pedal 10 times with the calipers off the rotors, the piston came out as expected. I put a c-clamp on an old pad and attempted to push the piston back in. I COULD NOT MOVE IT!! I am now wondering if the old combination valve is screwed up and not allow the fluid to return to the master. I did somewhat disassemble it and flush it with alcohol before I put it in the car. Does anyone rebuild these valves???

the pushrod should be about 1/8" away from the master... never touching at rest.

a quick test would be to unbolt the m/c from the firewall and see if the brakes release.

if the master pistons are pushed forward even a fraction of an inch, the brakes will drag and you wont be able to push the caliper pistons in as you describe. that would be my guess.


too bad you cut your rod. there are several deep hole masters which would have worked. now that you have a short rod, you need a retainer to make sure it wont fall out.

Just to clarify, there is no booster, this is manual disc. I can get another rod if anyone has a part number for a disc/drum manual master. My napa store could not come up with the deep hole manual cylinder. Otherwise, is this retainer you mentioned a part you can buy or do I need to fab something. I thought the rubber boot on the back was ment to keep the rod in place. I couldn't imagine that rod could drop out of there as the pedal was up against it's stop when I set the length of the rod.

i dont know why i said booster. I knew it was manual as my last paragraph had to do only with manual brakes.

67-76 corvette power, 67 firebird manual or power, and 67 chevelle manual or power all should have 1 1/8" masters available with deep hole. 67-76 corvette also should have a 1" manual brake deep master.

the retainer is not available aftermarket that i know of, but it is a safety feature. it is not likely to come out, but possible.

i edited my above post by deleting the word booster and adding the word firewall.

I'll try backing the rod off a little tomorrow and see if the pistons will then retract. Thanks for the advice. I know the master I have has a 1" bore, do you think the larger 1 1/8 would matter?

easiest would be to just loosen the master bolts for the test. otherwise you have to adjust the clevis, brakelight switch, and/or cut the rod some more.

1 1/8" from a manual brake '67 chevelle should give you close to the the stock ratio for single piston calipers, but the 1" is easier to push, though with slightly more pedal travel. The 1" was used with a different pedal leverage or calipers in stock manual applications. the only deep 1" i know of currently available is for the manual brake corvette.

i think some have even used a 15/16" from a 78 malibu. this is also a deep one i think, for manual brakes. i think it would be too soft for my taste, though i haven't driven one, so really cant say. the ports are reversed too.

if the rod length is the problem, and you can put some slack in it w/o replacing parts, stick with that if it works out.

OK, here goes what I did to make darn sure I couldn't lose my M/C's p/r b/c of using a shallow hole M/C!!

I bought a 1.5" copper pipe solder on cap and cut it about half down and then drilled a hole in it and then crimped it on the M/C's rubber cap groove by gently using a pri-k punch and small ball pein hammer that I am using AS a p/r guide to keep from ever dropping the p/r out of the M/C's piston hole IF the piston ever should stick in!!

Then I crawled under my dash at least 5 to 6 times to check the adjustment on my p/r AND also removed my M/C each time adjusting it to make DARN sure I had it adjusted just so it was like .050" AWAY from the piston!!

LIKE about an inch down at the pedal!!

DON'T under any circumstances run your car without a M/C p/r guide!!

If your p/r falls out for any reason, you are BRAKELESS!!!!!!

YOU CAN GET KILLED!!!

pdq67

Thanks for the advice, does anyone out there have a picture of the above mentioned factory push rod retainer?

Here is a photo of a second gen retainer cup and pushrod.

A shallow master cyl used for manual brakes can have the pushrod fall out of the depression in the piston IF the pedal ever returns faster than the brake piston does. IF this happens, the pushrod can fall down inside the MC bore and the piston is now pushing on the pushrod but the pushrod is not seated in the hole, the piston now can't return fully and vent system pressure to the reservoir, this would make the brakes lock the same as if the pushrod was a bit too long. The pushrod also can fall clean out of the master cyl if it is short enough and then you will lose ALL brakes.

There should be .010" clearance between the pushrod and piston. If you converted from power brakes, the brake light switch tab must be removed from the top hole on the pedal and the pushrod connected to there. Make sure the brake light switch is not preventing full pedal return to the rubber bumper. Power brake systems don't use a bumper so you have to get one for manual use. You also need a return spring and tab to connect the spring to the pushrod clevis pin.

Much thanks to everyone for the replys. I loosened the master the morning and found that the caliper piston would then retract. I must have adjusted the rod a little to snug. I backed the rod off and the brakes work great, (no drag). I think I will just try to obtain a deep hole master cylinder and the correct push rod unless anyone out there has a retainer they are willing to part with. Thanks again.