I think my master cylinder is shot. What to replace it with?

[dznucks]

I recently added rear discs to my '69 along with a wilwood adjustable proportioning valve. Ever since the brake pedal has not felt quite right. My first thoughts were air in the lines, so I have bled them over and over again. Just after the install it was easy to get the rear wheels to lock up and I could adjust the prop valve correctly.

Gradually the spongey pedal has gotten worse and no matter where the prop valve is adjsuted, I cant get any wheels to lock up or get a solid pedal. At times the pedal will feel normal only to go spongey as i come to a stop. Sometimes the pedal feels right and while sitting at a light it will go spongey. Other times it goes almost to the floor and has to be pumped back up. I'm pretty sure its the master.

The stock master was for a drum-drum application. On the autoparts websites they list the masters as for front drums, or front discs. Does the master really matter when it comes to drum/drum, disc/drum, or disc/disc? I thought this was only the proportioning valve the mattered to configuration.

I dont really want to get a reman'd unit from the parts store. What master cylinders do you guys recommend? This is a manual brake setup.

[eldogg]

I use Corvette master cylinders on all my 4 wheel disc builds. I only use Raybestos brake parts. Let me know if you can't get it figured out, I'm not that far away.

[dznucks]

Thanks eldogg. Do you have a part number for the Raybestos unit?


Did we meet at a restaurant car show thrown by FunkyTechMonkey last year?

[fleetsidelarry]

I still have the catalog that came with my Master Power Brakes kit, in it, one of the frequently asked Q's is "can I use my disc/drum master on four wheel disc brakes?"

the answer is no, it requires a true 4 wheel disc MC. (for one thing, discs require more fluid volume than drums)

another relevant Q is "how can I tell if my MC is bad?"

the answer is, you guessed it: spongy pedal and slowly sinking pedal

[dznucks]

Thanks for that info Larry. Thats what I needed to hear.

I'll get a raybestos unit on order as soon as I can find the correct part number.

[dznucks]

Is this the master cylinder?

http://www.carpartsdiscount.com/auto...ml?3593=188225

[gringoloco]

That looks like the right one. You want a 1" bore disc/disc unit. Power brakes would use a 1 1/8" bore.
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