What are the symptoms of a failing mc? There doesn't seem to be any fluid leak near the mount and my fluid level is full. What's happening is that the brake pedal starts to sink slowly about a second after I've applied the brakes slowly. The pedal will soften up under my foot. What's the cost/labor for repairing or replacing the mc? It's a 90 Si HB.

thanks,

 

Sounds like it. A good test is to do it with the engine off. Pump up for pressure, and hold. If your foot moves, the MC is bad.
280 for the part, (shop around, you never know, but they're brakes, do you trust a remanufactured one?)
The install is pretty easy, though you will waste a lot of brake fluid. A shop manual should be able to get you through it pretty well.
You need a bench bleeding kit, a vise, and a 10mm flare nut wrench, as well as a 14mm socket.
Good luck. It's not that bad, I did it in under 2 hours.

 

Well, seems like after the brakes hold, the pedal starts to gradually sink and stops about an inch or so off the floor, while the car's running. With the car off, the pedal holds firmly. No visible fluid leakage near the MC and the reservoir is full of fluid. I don't think any of the calipers or wheel cylinders are leaking but I'm not 100%. I know they were all replaced with new parts withink the last 4 years.

Someone suggested possibly the diaphram in the booster.

I don't want to buy any parts that I don't need. Mmm...

 

i think its the brake booser... to test it!

with the car off..,pump the brake pedal....it should firm up
hold the pedal down and it should remain as hard as a rock...
if there is any 'sinking' u have a problem..

also with the pedal still pressed, start up the car, the pedal should
go down...test it out..i say booster...
good luck!

 

I have a civic si master cylinder for sale, comes with the booster

100 plus shipping