brake system loosing pressure, post bleeding/master cyl install woes... Options

[7391420]

Help folks,

-Just completed a master cylinder swap, and then bleed on the system. I used a pressure bleeder/and second person stomping on the brake pedal process, for each caliper.

-after doing this, and with the pressure bleeder still set up, the pedal felt very firm, but as soon as I disconnected the bleeder, the pedal sinks to to floor....if I pump the brake a few times, pressure returns, but then after letting off the pedal for a few seconds, it falls to the floor again.....

-what's wrong!!

-I am not getting any leaks, and the system holds pressure under the pressure bleeder


-Adam

[Mueller]

bad mastercylinder would be my guess.........

brand new master cylinder???

Brand new does not mean anything, even they can be bad...........
a local 914 owner put a brand new master cylinder in his car and it was bad right out of the box, his symtoms sound just like yours....

[seanery]

or a bad bleeder nipple.

huh, huh I said nipple.

[rdracrdave]

Could be a bad M/C , But sounds like you still have air in the system.
Try bleeding the system the ' old fashoned way', and see if you get air out.

Dave

[mike_the_man]

I'd say bad master cylinder. The pedal shouldn't fall to the floor if there is air in the lines, it would just be very soft when pressing on it. One thing you could try is disconnecting the brake lines from the MC and plugging the outlets from the MC. Put some fluid in and try stomping on the pedal. It should be rock hard. If not, the problem lies with your MC, otherwise the problem is else where in the system. That should give you an idea, at least.

[7391420]

I was worried a bit about a bad M/C,

-it is a new unit, IE-i dont think it is re-built part...either way I'm sure it could still be bad...

-I got the car out on the road yesterday, (probably a bad idea...) and it's wierd, if I pump the brake like 3-4 times fast, it builds pressure and the brakes work fine.. but if I hit the brake, after not using it for a few seconds, there is no pressure....

-I dont detect any leakage still, and it doesn't appear to be loosing fluid from the resivour...


-I replaced all the bleed nipples with the speed-bleeder ones before bleeding, and they seem to work fine.. (although I dont see what makes them any different from the old ones...except that they are new and shiny and not old and rusted to hell..)


-Adam

[darisb]

Sounds like air in the system. I had a similar problem with the new master cylinder I put in my '67 Beetle. We bleed the snot out of it, using the two person foot method. Still, the peddle was nearly on the floor before the car stopped.

Air compresses. which is what is likely giving you enough peddle to stop when you pump it a couple times. Once you let off the peddle, the air expands again.

I finally bought a pressure bleeder from Motive Products (http://www.motiveproducts.com/). I was able to bleed the same car in about 15 minutes, and it's worked great ever since.

I would also check and make sure that the disc pads are adjusted properly (per the service manual specs.).

Regards,
Daris

[seanery]

Adam,

are you bleeding from the farthest from m/c then working closer?

I put a new m/c and brake lines on my harley and it took a looong time to get all the air out and the lines are longer on a car.

[r_towle]

I have had this problem before...I too replaced a master and could not get it to bleed out...

I had to bench bleed the master first...
I did this by connecting a line directly to the out port of the master for both the front and rear and sucked fluid through the master wiht a hand pump made for bleeding brakes....you kinda have to pump the brake pedal a little to line up the cylinder inside of the master to get all the air out...
Then reinstall all the lines and bleed out the calipers...should be fine.

Rich

[tat2dphreak]

I had a similar problem also... it would bleed fine at the calipers, but then lose pressure later... like air was getting in... figured out it was the prop. valve... ya gotta bleed it too... I didn't know.

[7391420]

Thanks for the help everyone,

-I think I have narrowed the problem down to;

-bad MC

-air in the MC

-N.F.C@#$@#

-I'm going to try the last bleeding the M/C bench style and see if that works


By the way,

do you need to bleed both bleeder screws on the front calipers? I did, but it didn't seem to make much of a difference?


-and yes, I did start with the right rear tire, then left rear, then right front, left front.

-Adam

[mike_the_man]

-I replaced all the bleed nipples with the speed-bleeder ones before bleeding, and they seem to work fine.. (although I dont see what makes them any different from the old ones...except that they are new and shiny and not old and rusted to hell..)

You should be able to bleed the brakes with one person with the speed bleeders. Rather than have one person stomp on the brake pedal and one open and close the bleeder nipple, you just have one person stomp on the brake pedal, the nipple lets fluid out, but doesn't let air in. They're supposed to work pretty slick. Have they been working for you?

[Dave_Darling]

do you need to bleed both bleeder screws on the front calipers? I did, but it didn't seem to make much of a difference?

Upper ones only--the air bubbles rise. The lower bleeders are, as far as I can tell, just there to get old fluid out of the caliper faster when you flush the system.

--DD

[7391420]

Mike,

They seem to work (although given the fact that I havent yet finished bleeding, I'm not so sure...)

so, are you saying that they have like a check valve inside so that fluid goes out, but cant get back in?

-this does seem to be the case, IE- you can see the fluid flow out when pressing the brakes (or when the system is under pressure-I'm using a motive systems pressure bleeder ) but fluid/air does not seem to be pulled back when the system is off pressure..

either way, for a couple of bucks, they are worth it, considering that the old ones can get destroyed from 30 years of hanging out on the caliper.

-Adam

[tdgray]

I agree with Rich, must bench bleed the new master cylinder. Had this problem on my MGA after a rebuild nothing I did made much of a difference until I did the old bench bleed.


Just my .02

Todd

[mike_the_man]

Yes, the speed bleeders have a check valve. They make bleeding a one man job. I've got to get some. I go through a lot of beer to lure my friends over to stomp on a brake pedal for hours on end. Much cheaper in the long run!