air in the brake master cylinder

porschenut · 06-19-2010, 07:53 PM

Ever since I replaced my brake master cylinder about 3 months ago, I haven't been able to get a nice firm brake pedal even with repeated brake bleeding with the Motive pressure bleeder. The thing is, I never filled the master cylinder with fluid before installing it - I forgot to do this and put it in dry. Is it possible there is air stuck in there that won't come out with a normal bleed procedure? Would it be a good idea to remove it, fill it up and reinstall it?

Another point is that I have not readjusted the actuating rod between the pedal and the MC. My understanding is that this would control the pedal height at which the brakes begin to bite, but probably wouldn't have anything to do with sponginess. Correct me if I'm wrong about this.

I'm really more concerned about the inability to get a nice firm pedal. I didn't used to have this problem before I replaced the BMC. However, the brakes work ok even on the track, so I'm trying to figure out where the trapped air is, and why it won't come out.

Flieger · Flieger's Garage

Yep, you should have "bench bled" the master cylinder. There is air in there that you cannot get without inverting. Maybe try tapping with a rubber mallet, like with the calipers, if you do not want to remove it again.

grahamkissack · 06-19-2010, 08:50 PM

I'm in the same boat with a spongy pedal despite over a dozen bleedings by all three methods and tapping the calipers. The pads are seated right up against the rotors are being rebuilt. I didn't bench bleed my new MC four years ago. I thought I knew how to bench bleed a master cylinder but I've never heard of "inverting". Don't you just push the rod and recirculate the fliud back to the reservoir. Is inverting tipping the cylider up on its end?

Thanks, Graham

Flieger · Flieger's Garage

I meant that there are cavities in the master cylinder which can hold air unless bench bled. The only way to dislodge the air with it hooked up to the system is to flip the master cylinder over and shake/tap it to get the air to move. In other words, the easiest thing to do would be take it off, bench bleed, and reinstal. Sorry for the confusion.

911pcars · 06-20-2010, 02:10 AM

Quote:

Originally Posted by Flieger

I meant that there are cavities in the master cylinder which can hold air unless bench bled. The only way to dislodge the air with it hooked up to the system is to flip the master cylinder over and shake/tap it to get the air to move. In other words, the easiest thing to do would be take it off, bench bleed, and reinstal. Sorry for the confusion.

I don't think this will work, otherwise one would have to perform this maneuver whenever air is in the MC. But it costs only your time to try. Inside the MC, there's only a horizontal bore and a couple of vertical holes connecting the bore to the reservoir. Pretty simple.

One could normally open a caliper bleeder screw and allow gravity to force out fluid and any air. I suspect you may have a torn piston seal that's allowing fluid to bypass the piston as it attempts to push fluid into the system. And since it's new, it could also be defective in some way.

Before you end up flushing new fluid from the system repeatedly, I'd disassemble the MC and inspect the seals to confirm all is well .... or isn't, then go from there. Remove one internal snap ring and all the parts are accessible (keep them in order).

Sherwood

greg bradbury · 06-22-2010, 01:08 AM

Gee,

I replaced my MC a couple of years ago and have had the same symtoms. The pedal feels spongy for the first say half of the travel then its solid loke as if there is air in either the front or rear of the split system. Bench bleedingdidn't seem to makeany sense at time time I put the MC as I thought the air would get in anyway as the pipes were not connnected. Ive bleed thru about 10 litres of fluid to no avail. Maybe I will have to give it a go.
Anyone else out there had any success retro bench bleeding?