master cylinder problem?

[ta240]

The first time I press on the brake pedal it goes down a fair ways and then the brakes grab some. If I let off and then press it again they go down just a bit and feel very solid. It never takes more than pressing it a second time to get it to work perfectly. They aren't awful with the first press but are much better with the second.

I read through the threads on here about the reaction disk in the booster and did the test where I remove the MC and pull the rod out and snap it back in and listing for a clink, meaning the disk is missing, but mine was a nice soft thud back into place.

I've bled the brakes several times with no real change.

bad master cylinder? Other ideas?

thanks,

Trevor

1973 240Z

[Z train]

If you are sure of no air in the systm,then the MC is by-passing internally.

[Bruce Palmer]

Look at it this way..... Knowing you have an extra pump in your future every time, don't follow too close or you know, POW, BAM!!

Had that problem years ago with big braikes with no dust shield. Always took an extra apply of the pedal to dry the pads before they were ready to be brakes. That extra gap between me and the car in front saved my bacon more than once. Some of this stuff just appears to be a problem. nyuck nyuck

[cmiulius]

Just an idea, remove the MC and prime it on the wise. Than install it and bleed the brakes. Hopefully work's for u. It did for me. Al these if u dont have a blow by in the MC.

[Marty Rogan]

The first time I press on the brake pedal it goes down a fair ways and then the brakes grab some. If I let off and then press it again they go down just a bit and feel very solid. It never takes more than pressing it a second time to get it to work perfectly. They aren't awful with the first press but are much better with the second.

I read through the threads on here about the reaction disk in the booster and did the test where I remove the MC and pull the rod out and snap it back in and listing for a clink, meaning the disk is missing, but mine was a nice soft thud back into place.

I've bled the brakes several times with no real change.

bad master cylinder? Other ideas?

thanks,

Trevor

1973 240Z

Did you change the calipers recently? Try this method:

If you still have a spongy pedal, try this trick that a racer taught me a long time ago. Bleed the rears properly first. Then on the right front caliper, remove the outside brake pad. Use some padded channel lock pliers and press the piston carefully into the caliper. It will travel farther than with the pad installed. BE very carelful when you push the pedal down, that you don't push the piston out past the seal. Do that a couple times till you get all the bubbles out. Replace the outer pad. Then remove the inner pad and repeat the process. Then move onto the drivers side and repeat.

Every time I would put new calipers on my race car, I could never get a firm pedal until I did this extra step. After doing it once, it was bleeding as usual.

Marty

[ajmcforester]

I had that problem with my current car so while the car is dismantled I took apart the MC, What I found is old crud from 39 years of use was creating leaks, so I cleaned it up. I hoped it fixed the problem, but I won't know 100% till the car is back together.

[ta240]

well, I've ordered a new one and will see if that takes care of it.

It was a reman one I put on a couple years ago and always kicked myself for not waiting for them to order the new one then.

A couple months ago I removed it and re-bench-bled it with little effect.

If it takes care of it I'll have to remember to stop double braking all the time

[ta240]

So I got my new (non-reman) Beck/Arnley master cylinder. Bench bled it, installed it in the car and as soon as I tried bleeding the rear brakes got a fountain of brake fluid shooting up out of the reservour.

[Zed Head]

If you had a lot of air in your lines and pressed the pedal, you would create a pressurized bubble in the lines that would then re-expand and push all of that volume back into the reservoir when you released the pedal. I'm betting that the pedal went way down and that the squirting happened when you let the pedal back up? There is a a lot of volume behind the disc brake pistons.

You probably did not have the bleed valve open far enough. Open the valve farther, leave the cap on the reservoir, and try again. Wipe that fluid off of your paint with a damp cloth too, or you'll end up with some stains.

[ta240]

it shoots out when I press the pedal.

It seems to have normal resistance for having the bleed valve open on the wheel cylinder and I've bleed a fair amount of fluid through it already.

[ajmcforester]

Do you have a vacuum pump that you could bleed the Master out again? I think you have air around a piston or some parts within the master.

The other thing to think about is a bad seal in the master. You push in which removes area for the fluid thus putting pressure down the line to the slave, which then expands pushing the pads out or in. The other thing the master does when you press the brakes is it cuts off the reservoirs so the pressure can be created. So fluid should not come out of the reservoirs when you press down; unless under pressure it is going around the seal. Now air moves better across the seals, so if you have a lot of air in the system you will see a short rise then a drop when the air is released. Not if all you are getting is fluid its going around the seals.

[ta240]

I spent quite some time bench bleeding the master cylinder before I installed it and kept going long after no air was coming out but anything is possible. I don't have a vacuum pump though.

It will shoot a nice solid stream of fluid up out of both reservoirs with even minimal pedal pressure, making me think there is a defect in the inside of the tube.

Edited November 13, 2010 by ta240