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Random Brake fade?!?

So I just replaced the master cylinder and power booster on my 1977 C-10, 250CI. I bench bled the master cylinder exactly according to the instructions that came with it. It came with plugs, instead of nipples. After a few hundred pumps in the vice, I achieved no more than 1/8" movement on the piston, which was what the instructions said. I then spent hours bleeding all the brake lines, starting with the furthest from the master, with a vacuum bleeder. I smeared a bunch of chassis lube around the bleeding nipples and vacuum tubing to prevent leaks. All lines were bled until all the old fluid was removed and the air bubbles stopped, this consumed almost 3/4 gallon of DOT 3. Took it for a drive today all around town, the brakes felt nice and firm, with a slight pull to the right when the brakes are first applied. Once home, I bled the front driver side(left), assuming that there was more air in the line due to the pull. After bleeding that line, the brakes felt firm, and good. I then started it up, and put my foot on the brake, only to have it fade to the floor. I press again, and had pressure. I cant get it to fade again, at least while parked, and I wont drive it until this is resolved. I’m worried that my new master cylinder is already leaking, and they leak right into the power booster which ruins the diaphragm. Any knowledge on the subject is appreciated, thank you.

More info to add;
I checked the fluid level after driving around town and I didnt lose any, but after the fade, the master was about 1/4" lower.

If the brake master cylinder drops in fluid level after operating the brakes, there’s leak somewhere in the brake system.


Did you get any air out of the left front line on the second bleed? It’s possible that you did, and when you started the truck and applied pressure to the system the brake pads on the left front finally moved out to the rotor, the pedal went down quite a bit because of the pads moving, and now you have a good system. If it was me I’d try driving it around the neighborhood, slowly, and stopping often, to see what it does. I predict nothing happens, and you are done.

Could that leak be at the master? I cant find any leaking fittings/lines that are leaking. I would think that much fluid would leave a puddle somewhere.

Remove the master cylinder from the brake booster to see if brake fluid is leaking into the brake booster from a bad master cylinder rear seal.


lol, dont know why that hadn’t occurred to me. Thank you, ill post results.

There is a little brake fluid in the power booster.

There shouldn’t be.

The rear seal on the master cylinder is leaking.


Thank you tester, that what i was thinking. At least it has a warranty.

A common “quick test” of the booster is to (1) with the key ON and the engine off, pump the pedal until it gets hard, (2) while pressing on the pedal, turn the engine ON. The pedal should sink some and feel softer. Failure of this response suggests a nonfunctional booster.

The pedal should NOT however sink all the way to the floor. Are you sure you didn’t feel the booster “quick test” response? Did the pedal actually sink that low? I’m not doubting your word, simply positing the question.

OP writes …

I bench bled the master cylinder exactly according to the instructions that came with it. It came with plugs, instead of nipples. After a few hundred pumps in the vice, I achieved no more than 1/8" movement on the piston, which was what the instructions said.

hmmm … A few hundred pumps? I’m not familiar with the C10 250 brake system – other than I like the looks of that truck – but when I’ve bench bled brake master cylinders either on my Ford truck or my Corolla, it has never taken more than a dozen pumps to get it bench bled. So I’m a little confused why it would take so many.

hmmm … when you put the MC into the vice, did you hold it in the vice so that the vice was squeezing on the cylinder? If so, that could be the problem. When bench bleeding you have to hold it in the vice by the flange, not by the cylinder. Other than than that, I can’t think of anything else other than as guessed above, the master cylinder is leaking into the brake booster.

Yes, The master was suspended in the vice level, and clamped around the the bottom of the housing. I was very concerned about the amount of bench bleeding that it needed, it could possibly be that “New and Improved procedure” that uses plugs instead of nipples and tubes. Or possibly a pressure leak in the master could cause this? I also viced it on a small unused bolt fitting on the master. Here is what I bought, you can see the “bolt” hole on the back of the master towards the power booster.

And yes mountain bike, the pedal sunk all the way, and then built pressure again right after. This is after driving all over town, scared the crap out of me.

If the chassis lube that was spread on the fittings has contaminated the brake fluid the master cylinder and all brake components with rubber in them are likely to fail soon.

The chassis lube was just used on the bleeder thread and nipple, after the brakes… No rubber components should be in contact with it. I didn’t use it on the master, that has flared fittings.

The plugs in the master cylinder ports are for shipping. Remove these plugs and use a standard master cylinder bleeding tool. I wonder if pumping the master cylinder for so long and possibly with insufficient fluid damaged the piston seals.

If this doesn’t get sorted out by conventional means you might consider the rear drum brakes. Rear drum brakes that are worn or badly out of adjustment can mimic a failing master cylinder and cause a mushy pedal and the symptom can vary in intensity.

And failing drum brake cylinders can mimic a failing MC. Why didn’t I think of the rear brakes?
Good suggestion.

It’s possible the MC box didn’t have all the parts it should have included. I’ve had that happen before. I think someone might purchase a MC, take it home to install it, then discover it is the wrong fitment. So they return it, but forget to put all the parts back in. The staff at the auto parts stores might not notice, since they don’t know everything that is supposed to be in the box. Maybe the required bench-bleeding adapters were missing for example.