WHITE 1974

Ive tried three different master cylinders two were crap from the get go. Bought a new one from a Corvette vendor. Worked perfectly...for about a month. First emergency stop and two miles later soft pedal and brake light comes on. no leaking lines. No leaking calipers. I had a friend whos a 30 year plus class A mechanic double check everything for me and sure enough I need a new master cylinder. its an L48 power brake car built feb 1974.

Peterbuilt

What test did the mechanic use to determine the master cylinder is bad?

Replacement M/C's have to be bench bleed.

Before you change it again you might bleed the system.
If the brakes improve but start to go bad again you may have a 'run-out problem.

WHITE 1974

It was bench blead,actually all of them were. First two didn't pass that test. Last one did and worked great for probably 200 miles. Thanks for the quick response Im worried I don't have the correct part. Ive replaced the master in a previous C-3 and several other old GMs never an issue.

HeadsU.P.

Before you go plunk down more cash on another MC. You need two short brake (test) lines (NAPA) with brass fittings on one end that fit the MC front port and rear port. (Neither are the same size and I can't remember the dia or TPI) If you are creative, you can re-use those test lines for bench bleeding another day.
Remove your brake lines from MC one at a time and install your "test line". Your test line should be closed off at the end some how. Note that some people use a bolt to plug the port but use caution as not to damage the flair inside the port. You should have a firm pedal with the test line in place. If not, then you know where the problem is. Keep in mind that now air has been introduced into the system and a complete bleeding will be needed.

New or rebuilt MC are not all interchangeable for C3s. Make sure you get not only the correct yr, but std brakes gets a different MC than PB. The difference is usually how far in the piston is inside relating to the distance from the brake pedal rod and the piston. That gap should be around 0.020.

WHITE 1974

Thanks hadn't heard of a test like that. have to give it a shot next week.

HeadsU.P.

Once you have a set made up, one of each size fitting, they come in handy for MC testing and bleeding. You can bend them back towards the MC for that if you have a tubing bender. You can put tremendous amount of brake pedal pressure on the lines where as those plastic bleeder kits always fail.

WTFizzit

Something else you might look into ----- compatibility of the pushrod between the booster and the master cylinder. Has to do with length, and if I recall correctly it can potentially be an issue if not matched properly. Sorry I can't give you the exact details, its been a long time since this fog-brain has touched on the topic. Do some searching, I'm sure you'll find something helpful.

WHITE 1974

That's what I was thinking. The last one I bought was a very good quality part, And like I said worked really well until my first emergency lock up. Got some really good ideas about testing here though Im almost certain it is a problem with the master. And unlike most my car is driven about 20-35 miles a day from April till late November, rain or shine and its usually driven a few miles a month even in the winter in our underground parking garage to keep things lubed up.