Master Cylinder kaput?

[GregL]

Problem two of my day, the master cylinder. Last time I drove my rig everything was fine. It sat for two weeks and today (after I installed my new lame water pump) I go for a test drive and my brake pedal goes all the way to the floor! Pump it once and it's all good, until I need brakes again, then same thing, the first press goes all the way to the floor, then the second press give me plenty of braking. No leaks, master cylinder fluid level is right where it should be. My local NAPA guy say the master cylinder is shot, it's bypassing internally, that explains no leaks, good fluid level, and pumping.

Suggestions on rebuilding or buying a new one and where? Do I have to bleed the system after install? Thanks.

[sbaumgartner]

Yeah, sounds like your MC is kaput. You can try a rebuild kit if the cylinder walls are not scored and in good shape. Might be best just to go with new to erase any doubt. John at Midwest has both. Be sure to bench bleed the master cylinder before you install it. And yes you'll need to bleed the entire system aftewards.

[MSeriesRebuild]

Problem two of my day, the master cylinder. Last time I drove my rig everything was fine. It sat for two weeks and today (after I installed my new lame water pump) I go for a test drive and my brake pedal goes all the way to the floor! Pump it once and it's all good, until I need brakes again, then same thing, the first press goes all the way to the floor, then the second press give me plenty of braking. No leaks, master cylinder fluid level is right where it should be. My local NAPA guy say the master cylinder is shot, it's bypassing internally, that explains no leaks, good fluid level, and pumping.

Suggestions on rebuilding or buying a new one and where? Do I have to bleed the system after install? Thanks.

The check valve in your M/C has failed. I wouldn't recommend a rebuild unless sleeving with brass or stainless, new is reasonable in price & easy to get. Yes you will certainly have to bleed the system after installing.

[GregL]

That's great info, thanks. I guess John is the go-to guy. I ordered a fuel filter and some other stuff form him earlier this week, sounds like another call is in order.
One more question, I understand how to bleed the system, I've done it on other vehicles before, but what does it mean to bench bleed the m/c before I put it on? Thanks again.

[sbaumgartner]

If you Google "bench bleed master cylinder", you'll find a bunch of sites with pictures like this one: http://www.superchevy.com/technical/cha ... index.html

Basically, you want to lock the MC down in vice, fill it with brake fluid and then pump it by hand with a wooden dowel to get all the air out before you install it in your truck. Create a closed system by leaving the fill plug off and running a clear hose from the MC outlet back into filler opening. As you pump, brake fluid and air bubbles will circulate through the hose. Eventually, the air bubbles will lessen and then disappear.

Charles has some sage advice in this post. Don't pump with full strokes, only 3/4 or less.

http://www.powerwagonadvertiser.com/for ... r+cylinder

I think this applies more for used cylinders than new, but I've always done it on new too. I just use short strokes and avoid pushing the piston in too far.

--Steve