Bleed after new Master Cylinder?

[Green79Scottsdale]

So, got in the truck to move it out of the garage the other day. Push the brake pedal and it stops in the usual manner, for a few seconds anyway. It proceeds to slowly go to the floor. Damn. Get out to look for leaks, nothing, front or back. Damn. Master cylinder must be bad.

My first question is, after you put a new master cylinder on (after bench bleeding of course), do you need to proceed to bleed the entire system? Conventional wisdom tells me yes, but I have heard of bleeding it at the fittings on the master cylinder. Anybody have experience doing this? Can I get away with just bleeding the fronts after a master cylinder replacement?

I ask because during my search for a leak I noticed the condition of my rear brakes (14FF) is something to be desired. Around the bleeder screws is all rust, and I am pretty sure if I touch them, they are going to break off. If that happens I will be forced to pull the axles to fix it, which is going to lead me to do a disk brake conversion. I do not want to do all this at this time as I have other things that require my time and money.

What says the GMSB community?

 

[cmull]

I would think that you would need to bleed the rear brakes if you want them to be right, air will get in the lines.

 

[Green79Scottsdale]

The whole system really should be bled, just because it hasn't been done in forever. I would like to get away with putting new wheel cylinders on the back, but I really would like to avoid pulling axles at this time if possible. And I am afraid I will find things that should be replaced while I am into the rear. At a certain point, it could almost be cost effective to do the rear disk swap instead of rebuilding the rear drums. Remember, I have no leaks anywhere, so I honestly don't know where air is getting into the system at.

 

[chengny]

Bench bleed it and keep the plugs in (the plugs are solid now) until everything it made up - except the brake lines to the master.

Pull the bleeder plugs (one at a time) and quickly make up the brake lines to the master. But here's the trick:

Leave the fittings slightly slack - just enough that a small amount of brake fluid escapes when the pedal is depressed. Pump the brake pedal slowly and watch the threads at the fitting.

A few bubbles will come out and then only brake fluid. When it's just brake fluid, have your helper apply the brakes and then tighten the fittings permanently.

You might have a tiny pocket of air left in there but it will quickly rise up into the master and bubble out in the chambers.

Remember the brake fluid doesn't really move anywhere - it just kind of ebbs and flows and stays in the same place. The air will want to rise to the top, not go down into the pipes.

 

[Green79Scottsdale]

Thanks chengny. Makes sense. I know a small amount of air will get into the system at the fittings, there is no avoiding that. And I started thinking, air goes up, so there must be a way to get that air out by the master. That makes me feel better about things.

 

[Green79Scottsdale]

Now... do I replace it with a "stock" master, or is there an "upgrade" I can do while I am replacing it anyway?

 

[chengny]

I always check rockauto first - to see what they have for closeouts. Many times if they are cleaning out a warehouse you can get good quality stuff cheap.

For example:

RAYBESTOS Part # E105891
New Master Cylinder 3/4 TON and 4 WHEEL/ALL WHEEL DRIVE,SUBURBAN - 13" BRAKES [Wholesaler Closeout - Private Label Pkg. - 30 Day Warranty] (Only 1 Remaining)

$47.89

 

[GTME94]

Bob,
I don't remember, is you truck a camper special? I just replaced my master cylinder last month. With mine being a camper special it has a different brake system than just a normal 3/4 ton. I was at the Zone and Advanced. Got the wrong one the first time. Finally got a good counter guy at Advanced and we looked at the pictures of the part on his screen before ordering the correct one. There were 3 or 4 options for my '86 C2500. If you happen to know the brake option code from your RPO list that will help.

 

[Green79Scottsdale]

Yeah it's a camper special. When I rebuilt the fronts a couple of years ago, the old timer I got the parts from asked what brake package I had. At the time I didn't realize how many different configurations were available! I need to look again to be sure what I have. At least they are a "cheap" part.

 

[GTME94]

I think my master was only $24

 

[Green79Scottsdale]

Just ordered the master for my JB7 brake system from Rock Auto for $39 total. Every parts store around here wanted $70+! That is, if they could understand either the J55 or JB7 codes I was trying to get through to them.