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JohnIL · 10-03-2022, 10:39 PM

One day, my son and I took the truck out for a drive in the country. My excuse for the drive was to give my son some practice driving a manual transmission. The C10 has been his first opportunity to learn, he needs more practice with the third pedal. As you would expect it was a slow drive with lots of stopping and starting. We only went about 15 miles. But, when when we pulled back into the driveway, I could smell the brakes. Not good. All four drum brakes were locked up solid. If it were a single brake, I would suspect a bad wheel cylinder or a stuck brake shoe. But, with all four brakes locked, it almost had to be a problem with the master cylinder. After letting the truck sit for about an hour, the brakes had released and the truck would roll freely again. This supported my master cylinder theory. The brakes either released as the truck cooled down, or the fluid pressure leaked back toward the master cylinder. So, I ordered up a replacement for the single-reservoir master cylinder. I hated to pay for a new master cylinder that will get replaced when I do the power disc conversation, but I don't plan to dig into that until next Spring. So, money was spent on a temporary master cylinder to get me by until the big conversion takes place. On the plus side, this was the easiest master cylinder I've ever replaced. There's only one brake line to remove and reinstall. And, the fitting on that line even cooperated. The fitting itself is a little bit rounded over, but once it broke loose, if threaded right out of the old master cylinder and back into the new one. The truly amazing part of the replacement is that no (or at least very little) air got into the system. I bench bled the master cylinder before I installed it. Once it was on the truck, I got one burp of air from the bleeder on the top of the master cylinder and the brakes seem to work very well. The pedal feel is MUCH better than the original master cylinder and the brakes will lock up on dry pavement. If there is any air in the lines, I'm sure it will show up with enough drive time. We'll have to take it out for another stop and go test drive to make sure the brake lockup problem is fixed.

Update on Brake Master Cylinder
So, I took the truck for another test drive. I went of five miles. When I pulled back in the garage, the truck came to an abrupt halt. That's right, the brakes were locked again. And, the brake pedal was a hard a rock. What the heck is going on? I have a brand new master cylinder and the blasted thing locks up, just like the old one. Then, I had a stroke of brilliance. I stuck my head under the dash to lay eyes on the brake pedal, push rod, and the back of the master cylinder. You'll never guess what I found. The jamb nut on the push rod adjuster was lose. So, each time I pushed the brake pedal, the push rod turned a tiny fraction, which made it get longer with each stroke of the pedal. This put constant pressure on the master cylinder, especially when everything under the hood (including the master cylinder) was hot. I backed the push rod off a couple of turns, creating about 1/4" of slack in the brake pedal and tightened up the lock nut. Then, I went for another drive. All good now. The brake pedal feel is different now that there is some "take up" travel before the master cylinder engages, but the brakes are nice and strong. And, best of all, they don't lock up when they get warm!

I question whether or not there was ever anything wrong with the original master cylinder, but at 57 years old, it was time for it to go anyway.

10-03-2022, 10:39 PM	#12
JohnIL Registered User Join Date: Aug 2022 Location: Central Illinois Posts: 112	Brake Master Cylinder One day, my son and I took the truck out for a drive in the country. My excuse for the drive was to give my son some practice driving a manual transmission. The C10 has been his first opportunity to learn, he needs more practice with the third pedal. As you would expect it was a slow drive with lots of stopping and starting. We only went about 15 miles. But, when when we pulled back into the driveway, I could smell the brakes. Not good. All four drum brakes were locked up solid. If it were a single brake, I would suspect a bad wheel cylinder or a stuck brake shoe. But, with all four brakes locked, it almost had to be a problem with the master cylinder. After letting the truck sit for about an hour, the brakes had released and the truck would roll freely again. This supported my master cylinder theory. The brakes either released as the truck cooled down, or the fluid pressure leaked back toward the master cylinder. So, I ordered up a replacement for the single-reservoir master cylinder. I hated to pay for a new master cylinder that will get replaced when I do the power disc conversation, but I don't plan to dig into that until next Spring. So, money was spent on a temporary master cylinder to get me by until the big conversion takes place. On the plus side, this was the easiest master cylinder I've ever replaced. There's only one brake line to remove and reinstall. And, the fitting on that line even cooperated. The fitting itself is a little bit rounded over, but once it broke loose, if threaded right out of the old master cylinder and back into the new one. The truly amazing part of the replacement is that no (or at least very little) air got into the system. I bench bled the master cylinder before I installed it. Once it was on the truck, I got one burp of air from the bleeder on the top of the master cylinder and the brakes seem to work very well. The pedal feel is MUCH better than the original master cylinder and the brakes will lock up on dry pavement. If there is any air in the lines, I'm sure it will show up with enough drive time. We'll have to take it out for another stop and go test drive to make sure the brake lockup problem is fixed. Update on Brake Master Cylinder So, I took the truck for another test drive. I went of five miles. When I pulled back in the garage, the truck came to an abrupt halt. That's right, the brakes were locked again. And, the brake pedal was a hard a rock. What the heck is going on? I have a brand new master cylinder and the blasted thing locks up, just like the old one. Then, I had a stroke of brilliance. I stuck my head under the dash to lay eyes on the brake pedal, push rod, and the back of the master cylinder. You'll never guess what I found. The jamb nut on the push rod adjuster was lose. So, each time I pushed the brake pedal, the push rod turned a tiny fraction, which made it get longer with each stroke of the pedal. This put constant pressure on the master cylinder, especially when everything under the hood (including the master cylinder) was hot. I backed the push rod off a couple of turns, creating about 1/4" of slack in the brake pedal and tightened up the lock nut. Then, I went for another drive. All good now. The brake pedal feel is different now that there is some "take up" travel before the master cylinder engages, but the brakes are nice and strong. And, best of all, they don't lock up when they get warm! I question whether or not there was ever anything wrong with the original master cylinder, but at 57 years old, it was time for it to go anyway.