Most bizarre brake problem...sucking in air...

BGCarrera32 · 07-28-2003, 06:16 AM

Just thought I'd post to get any ideas before I tackle my brakes again...

Here are the details...

Basically rebuilt half of my brake system a few months ago. Rebuilt all the calipers, changed to stainless steel sheathed hoses, changed the 2 small hardlines up front from the calipers to the hoses, turned all rotors to spec, Mintex pads, new feed hoses from reservoir to master cylinder, flushed, bled, Castrol GT LMA fluid.

Here's the problem, its been happening ever since I did all the work a few months back:

You can bleed the brakes completely, get firm pedal, all is good. Stops fabulous. Drive about 12-15 miles, repeated stops, did a few hard stops from 80mph...I lose about 1/8"-1/4" fluid from the reservoir out the overflow vent tube (verified). Then the pedal gets progressively mushier... I bled the brakes again and noticed I'm getting a bunch of air out the drivers side rear caliper that I just rebuilt. Is it possible that air could get sucked in ever so slowly somehow, but not lose fluid out the caliper? Everything on the exterior is bone dry.

I know how to bleed brakes, the fluid is clear as a bell, and I guarantee I'm not boiling fluid after 12 miles. dtfastbear has posted a thread on the same problem before...I'm wondering if there are any new ideas out there before I rebuild the caliper (again). I'm not convinced its the master cylinder. I think air is getting sucked in and displacing fluid out the over flow tube, which continually displaces the level at each pedal application.

Your thoughts appreciated....

-BG

BGCarrera32 · 07-28-2003, 09:41 AM

bump

Thomas Owen · 07-28-2003, 10:19 AM

When you bleed do you get air at all calipers or just the one? If the problem is 'upstream', like the reservoir or master cylinder, the air will appear at more than one caliper. If only one caliper then that points to that area only in my opinion. I must say, it sure sounds like a master cylinder to me.
Good luck,

BGCarrera32 · 07-28-2003, 10:47 AM

Well, it would appear as though it is only the one caliper. I hadn't bled since I initially did all the work. I was getting a ton of air out of the driver's side rear caliper yesterday. I also use speed bleeders, and thought maybe I was doing something wrong during bleeding...so I switched them back to the old bleed screws. But I don't see how they could be causing a problem. I got essentially nothing out of the other calipers.

I'm just not seeing how I could be sucking air past, say, the caliper seals, but losing no fluid there...

Noel · 07-28-2003, 01:10 PM

I would guess the Master Cylinder("MC"), also. Years ago I had a 300zx that did the same thing when it's MC died. Let me guess, you pumped the brake pedal to do your bleeding? This is very common with older master cylinders. The pumping from the bleeding makes the MC seals travel past the normal point of travel and it chews them up. I replaced my MC on the SC about 6 months ago. (Also, I ended up breaking one of the plastic niples that attached the resevoir hose to the MC and had to order one from Porsche. If this car is your daily driver, I would suggest ordering a couple of these first. (Only around $3 per piece) It is not a very hard job, but I recommend a Motive pressure bleeder for the bleeding duties.

Good luck,

Noel

dtfastbear · 07-28-2003, 03:18 PM

I sent a private message to BGCarrera on this topic, but I thought I share some of the content in the thread, too.

One thing to think about. If it is the MC, then just turning the car on (to get the brake assist involved) and pumping on the brake pedal should duplicate this behavior, right? In other words, he'd see the brake fluid overflow without the car ever moving.

If this is indeed the case, then I believe that you could safely say there is definitely a leak somewhere. The comments about the MC seals are definitely one possibility. The other is that a seal isn't "seated" correctly in one of those calipers that you rebuilt.

One thing that I just thought of when considering your most recent post - you said this is that you said this is the first time you've bled since you did the rebuild?? It very well may be that a big bubble finally worked its way loose somewhere in the system. Re-bleed and then see if it happens again.

If with the car standing still you can't reproduce the behavior but then you go out and drive the car and the pedal gets all mushy, I'm thinking that you may have a problem similar to mine. A rear wheel bearing was shot and the rear wheel was actually moving around on the stub axle. This was pushing the pads and pistons back into the caliper (pushing out brake fluid that is displaced) and causing the mushy pedal as the first couple inches of pedal travel are now getting the pads back onto the rotors instead of applying pressure.

Thoughts?

Dean

BGCarrera32 · 07-29-2003, 05:32 AM

I've since re-bled everything, driven a few more miles, and still have the same problem. I will rebleed soon after I give it a few miles to suck in some more air...I'm thinking the caliper I was bleeding all kinds of air from is amiss and it wasn't sucking air past the threads of the bleed screw like I thought. I think something went awry on the rebuild of that caliper...if the seal flexes and sort of "self energizes" when the brake pedal is applied, and when the seal flexes back to retract the piston it probably creates a low pressure condition and may very well be sucking tiny amounts of air in past a nick in the seal or piston over time. For the fluid to keep dumping out the vent tube it must be getting displaced by something, and that would be air. If the master cylinder was at fault you'd see air at all calipers, but I don't. My rear wheel bearings are brand new and there is no play (checked again yesterday), the fronts are o.k. too.

I'll let ya know what I find out as I go along.

-BG

stomachmonkey · 06:43 AM

I you think the caliper is the fault and you are going to remove it why not just swap it with the other side, bleed, drive a bit and see if it sucks air in the new position.

Probably less work than rebuilding it again only to find it is something else. And if it is the caliper, most likely, you still need to rebuild it anyway.

Worth a shot.

Scott

BGCarrera32 · 07-29-2003, 06:51 AM

Sounds good, but if I swapped sides the bleed screws would be upside down and I'd never get the air out from the swap...

Noel · 07-29-2003, 06:57 AM

I'm not sure the left and right calipers are interchangable. The bleed nipple would not be at the highest point of the caliper if they were switched and you would not be able to get the air out.

Try removing the caliper piston dust cover to see if you have any fluid leaking past the piston seal.

BTW, how old is your mastercylinder? If it is pretty old, you might want to replace it anyway. If it turns out to not be the problem, then you have a new one in place and will never have to worry about it again. They are only about $150. Just a thought.

If money is an issue, then just tear the caliper apart again. It won't take THAT long, since you already cleaned everthing the first time. Replace the seals with new ones and check for nicks on the pistons.

It's got to be one or the other.

Noel