AO

If it's a pressure point problem, could the vacuum booster be suspect?

dr bob

The pressure gauge won't tell you anything you don't know already. If pushing the pedal down further makes enough pressure to engage the brakes, you'll see that pressure on the gauge when you engage the brakes. It's one of those fringe benefits of hydraulic systems.

I have a reservoir cap with a quick-disconnect fitting on it. It's a regular air line fitting threaded into a bushing glued into the cap. Iknow the bushing seal will be OK for vacuum, but have a concern with the air-line female connection holding. Otherwise I'm ready to pull vacuum on a brake system I think, using the AC system pump I have. It would need a trap to protect the pump of course. So then the question is how to get fluid into the system under vacuum? We know from the Miti-Vac tool method that the bleeder screws leak air around the threads. I'm not sure I'd want to use grease or other flowable sealant on the threads since vacuum will try to draw that into the system when you unseat the bleeder valve.

I also have a concern that serious vacuum will draw air in from the pedal end of the MC piston seals. That migt not be an issue for the pump but it will limit the vacuum available. I wonder what would happen if I just tried to pull a vacuum on a system that's full now, just to test the MC seals. It would probably be OK since any air drwn thou will rise to the reservoir top anyway, or will be easily removed via the MC bleeder port. Drawing a vacuum will cause air bubbles to expand and theoretically migrate towards the vacuum end even in a system with fluid in it. Air gets real big under high vacuum for some reason, just need the bubbles to make it to the MC reservoir I guess.


Thoughts from the experts? Bill, do you want the adapted cap and an air-eductor vacuuum pump to play with?

Bill51sdr

Not the "Bill" you were looking for here...

I use an A/C vac pump in conjunction with a Mitivac bottle and its fittings connected to the pump. I have done this for years with all my cars, including 2 ABS equipped 928's with great success. I connect the Mitivac tube to the calipers and regulate fluid flow with the bleed nipples on the calipers. You just have to be constantly aware of the fluid livel in the MC reservoir as this method will drain it quickly. This is a very easy one-person method but having an assistant to watch the fluid level for you is certainly helpful.

This method was the way I finally got a nice, firm pedal on my 86.5 when I had to replace the MC on it.

SharkSkin

Bob, I don't think you can pull a vacuum via the reservoir cap without finding a way to seal this up:



Even if you did seal that up I think you run the risk of having the reservoir implode. That's why I suggested in post #23 making a manifold with shutoff valves that connects in place of the reservoir. Also drilled-out bleeders with shutoff valves (mentioned in the same post) would eliminate the issue of drawing air in past the bleed screw threads, and would provide full control over fluid flow to each circuit. Having a manifold/reservoir would allow you to vac the whole system at once. Other than the reservoir and the extra bleed nipples this is all hardware-store stuff.

Bill Swift's approach sounds reasonable but IMHO the more vacuum we can pull on the whole system, the greater our chances of success.

Charles Parkinson

Hi Guys,
I just finished re installing the braking system on my S4 project and have a few suggestions in case you have not already tried them. As background, all brake pipes were replaced and calipers, ABS pump and master cylinder had all been off the car for at least 12 months - so it should be a worst case scenario.

After reading your experiences I was quite nervous and was super pleased when I ended up with a pretty much perfect feeling brake pedal after about 1.5 hours work. I used a simple vacum bleeder as they give very good results in my experience and avoid some of the problems with pressure bleeders.

First Step: I adjusted the brake pressure rod as per Section 46-10 of WSM. I think this is critical and assume you did this as the master cylinder was changed. If not, this could be why the pedal is only firming up toward the end of its travel.

Second Step: I connected my vacum bleeder to the master cylinder bleed screw and pulled a lot of fluid through, in combination with pumping the brake pedal until I had a nice flow of fresh fluid.

Third Step: I bled each caliper as per WSM order with the vacum bleeder. Procedure I use is to pull a vacum, open the bleeder and suck plenty of fluid through, then I leave the vacum bleeder connected but not sucking and pump the brake pedal a few times to push some fluid through. Then I switch the vacum back on and pull more fluid through and close the bleeder whilst still under vacum.

Fourth Step (optional): Let it sit for a while then re bleed each caliper.

Overall I ended up with 1.5 litres of fluid in the vacum container, so the total bleed took about 2 litres.

Hoping this helps.

Cheers, Charles in Oz.

Nicole

Thanks for the write-up, Charles!

I'll let the mechanical experts here determine, how this could be tried on my car.

Mrmerlin

sounds strange but was the piston depth checked/ pushrod on the booster, prior to installing the new MC?? it might be part of the problem

Nicole

How can the adjustment of this change, just from removing calipers and fluid from the system?

Remember: the brake feel was not the reason for the whole exercise. The pedal felt perfect before.

SharkSkin

Sounds like it's worth a try, if that doesn't work maybe a full vac-down of the system would be next.

Mrmerlin

Nicole its possible that the new piston on the new MC has a deeper hole and thus the booster push rod might have to be lengthened to compensate, OR the pushrod could be too long now (doubt it)

Like i said B4 it is now time to remove all of the new parts you installed and check them, since the brakes were working fine B4 the bleeder snapped off, and then once the NEW calipers were put on the problems began, and then we have been chasing our tails, by replacing parts and doing all kinds of extra work to fix a problem that we still have not identified

Bill Ball

If so, this certainly could be the problem. I wasn't there for the master cylinder replacement, but I think no adjustment was made to that rod. I thought that was only done if the brake booster was replaced. I'll read over that section again.

We've bled the brakes anough different ways and times that I don't think air is the problem any more.

Bill Ball

That is not true. The problem began when Nicole changed the brake lines and rebuilt the calipers, and then we found gobs of air. The change of the front caliper came later. The calipers are pretty simple. They can leak, they can stick, but that's about it. I don't see how they can account for the way the brake pedal is behaving.

Nicole

The rebuilt fronts and one rear caliper were installed at the same time as the brake lines; the other rear caliper had been rebuilt earlier with no ill effects.

Front calipers were rebuilt using Zeckenhausen rubber parts and old pistons; rear calipers were rebuilt with genuine Porsche BREMBO rebuild kits that included the pistons. The new brake lines are genuine Porsche as well.

When we replaced the master cylinder, there was nothing to adjust that I know of. I still have the old one in the garage, and will take another look, though. I remember we compared old and new, and everything looked exactly the same from the outside.

We have never lost any fluid since all this was done.

Either way, this last experiment did not change anything in the pedal feel. It still feels long, soft, and mushy - the opposite of what Porsche brakes should feel like. Something in that system must be soft - I can't imagine what that could be other than air.

AO

Maybe a bad brakeline that's balooning?

Mrmerlin

Damn I am puzzled, none of this makes sense.
Start over, remove all of the parts and check every one...........at this point what do you have to lose.
I still like the idea of replacing the calipers with the originals