I have a set of older (97 or so) PM 4 piston brake calipers on my bike which just got rebuilt today with new o-rings, seals and pistons. Also new is the rear reservoir, rear brake master cylinder and front brake reservoir and master cylinder. I did not replace either the front or rear brake lines, YET.

I have cleaned out everything (using brake cleaner). Brake lines, calipers, you name it, I cleaned it.

I can not get them to hold pressure. I can get air free fluid through the bleed screw, but it does not hold pressure. If it makes a difference, I am bleeding the old fashioned way. Pump and crack bleeder, repeat process.

My first thought was the master cylinder pistons. But, if they are new, why would that be the problem?

Am I overlooking, or thinking too much on this simple process? I have never had any trouble bleeding brakes before. A dozen or so pumps, and it was good to go.

Anyone have any suggestions or comments?

Thanks.

 

You do have the cover loose? Most people do not realize the cover is not vented. That diaphragm bellow in the gasket cover with the vent to the top side is how system works. Since you are starting from totally scratch, let it just gravity bleed first. Then break they banjo on the master. Be real careful if not using dot 5. What did the cylinder bore look like? Most new rebuild kits come complete cylinder and piston. What did you use. The quickest way is to do the master with a tube to the reservoir. Then the line hookup.

 

Yes. Both covers were loose, even off for a while. I started out by back bleeding and got most of the air out that way.

Using DOT 5 fluid.

By checking the cylinder bore, I assume you mean the front brake, right? If so, the whole assembly (lever, reservoir, and master cylinder) was purchased as a whole. No assembly required.

As for the rear master, it was a complete master kit. Just install it and "go". As it only fits in the reservoir one way (due to the tab) it was kinda hard to not get it right.

Thanks for your quick response!

 

Bleeding brakes is a frequent problem in my long experience! They will either do it with little protest, or put up a real fight. Persistence pays and you can find help in the Stickies in the DIY Brakes forum.

 

@grbrown:

I actually used one of your tips. Turn bars all the way to the left so that the m/c is at the highest point.

There was a bunch of air that came out that way! Thanks!

And you are right about it going really fast, or taking forever. Mine are going the latter route.

Thanks for the help, guys!

 

Tried using a vacuum pump? Pretty cheap, most place & the only way I've found to reliably bleed braided lines w/o cursing up a blue streak & throwing things.