dr bob

Vanster, using the hand pump is a ot of exercise with reaktively small benefit, at least it was for me. You might do as well just pushing fluid from caliper to MC with your power bleeder.

I was looking to pull all the air out of the ABS and other "local" high spots with the real vacuum pump. By then I had pushed fluid from the rear calipers through each of the lines to the T that's on the bulkhead forward of the battery, and then used the vacuum and power bleeder to evacuate and then fill the ABS unit from the rear caliper area.

Some master cylinders have a separate bleed port on the MC for the front piston. Hook up a short hose to that and put the other end back in the reservoir. The MC is slightly nose-high, and the brake line ports are low on the cylinder. That leaves a pocket where air can sit.

ATE published a guide to bench-bleeding their master cylinders prior to installation. I have it around here somewhere and need to scan it and share with the group. In a nutshell, the MC is clamped in a vise or similar, with short lines connected to each outlet port. The other end of those lines drains into the reservoir. Using a screwdriver, you get to push the pistons through a few full cycles. Then install the MC. And finally remove those short lines and immediately connect the lines from the car, which are presumably still full of fluid. When done correctly, no need for further bleeding they say. ATE also supplied a cute little set of plastic threaded fittings that fit the MC ports and allows a small hose to connect to each port to facilitate the bench-bleeding process. You can buy steel brake line cheap and just bend it into the return shape needed, might be better than securing the plastic lines and all.


In the end, you are working with the idea that bublles will eventually end up in the high spots in the system. The caliper bleeds are at the highest points on the calipers, but the lines slope up from the calipers. Follow them to each high spot, even if it's just a little higher than than the surrounding tubing (the rear T on the bulkhead...), and bleed the air out there. On my ABS car, the ABS unit sits above the MC height, so air was gathering in it. Hence the vacuum method for removing the air from that part. Otherwise the high points are easy to find and bleed.

Chalkboss

I have done the bench bleeding for a master cylinder on several cars and it works great. You have to be careful not to push too hard or too far with the bore or you risk damaging internal seals. Just short easy pushes will get it done.

vanster

I spent a few hours yesterday trying to get the bubbles out using the vacuum and the passenger rear seemed to be OK the drivers rear is a mess. Passenger right front is getting better. I have gone through 2 bottles of Dot 4.
What happens if I use the pressure bleeder at the same time?
Also, since there is an order to this, if I get one, do I just have to stick with the next until I get a satisfactory result before moving on to the next?
My hopes of going to Monterey are getting slim.

Pfc. Parts

Maybe someone else will comment, but I used a collector bottle on the calipers and after I'd moved all the old fluid out I started re-cycling it through the power bleeder. It saves fluid and I don't think it has much of a chance to pick up water after the old fluid is out. Using a different color fluid helps with this; if you have amber in the car, use blue for the flush.

I can't understand how using a power bleeder with a vacuum pump would be different from using the power bleeder by itself.

My procedure was to bleed the front driver's side first until I had no air, move to the front passenger until there was no air, then the rear driver followed by rear passenger. Remember to tap the calipers with a rubber or wood mallet while bleeding, that will help knock loose air trapped in the calipers.

After each full rotation around the car I pumped the master cylinder a few times then repeated the process until I got no air from any of the calipers and a solid pedal. If it's still spongy after that I wouldn't know what else to do other than replace the master cylinder (and slave if you have a 5 speed).

PS: Sorry for the mixed metaphor. There's no slave on the brakes. I'm in the process of replacing the clutch hydraulics on my sons 1.8L A4T

dr bob

Vanster - a little silicone grease on the threads of the bleeder screws will help eliminate air drawn in by vacuum around the threads. I have a bit of "high vacuum grease" used for lab equipment, but almost anything (like Vaseline?) will likely do the job fine.

The ATE instructions provide for bleeding the rear section of the MC first when you are pedal-bleeding; That corresponds to the front brakes on the car. The two pistons are coupled hydraulically unless there's a failure/leak in the fronts, at which time the rear piston pushes more mechanically on the front piston. In reality it's one piston in there with two sections, with what amounts to a check-valve between them. So front brakes get "pedal priority" I guess. When power-bleeding, it really doesn't make a difference whether you do fronts or rears first, but you still want to do the furthest of the pair first, especially in the front. For the rears, the difference in line length is inches, and they are common to that T on the rear footwell bulkhead.

Pfc. Parts

Forgive me, I'm confused again (it's my nature).

IIRC, I was chastised within the past 6 months (on this board and topic) for following the "furthest caliper first" protocol, and instead referred to the WSM, which doesn't really have an opinion on the subject. I was told that these cars had a bleeding sequence exactly opposite the standard procedure.

I'm now receiving further conflicting information, however it still doesn't sound completely definitive. Is there a correct bleed sequence? Will the real bleed sequence please stand up?

vanster

I have spent the better part of the afternoon working on this. At one time I thought was getting some peddle.
Interesting that now the right front will not bleed at all!
I tried with incoming pressure, then I tried just vacuum, then I tried both at the same time. I tapped on the caliper. I bled the clutch slave cyl.

ammonman

On page 32 of the 928S w/ABS Infotechnik publication step #8 for installing the ABS hydraulic unit states "Bleed the brakes in order of stepped brake master cylinder, push rod brake circuit (front wheels) and intermediate piston circuit (rear wheels) and check for leaks. Bleeding procedures are the same as for cars without ABS."

Mike

dr bob

See my previous post. When pedal bleeding, you need to do the front right, then front left, then rear right, then rear left. When pressure bleeding, pick front or rear to do first (doesn't really matter), then do the right side first of those and then the left side.

The big idea on the right side vs left first is getting all the air and old fluid out of the longest line, and hardening that line so when you do the short side there's less chance of pushing dirty fluid into the longer line.

This guidance applies to 928's with front/rear divided system. The diagonal system on early cars may have different needs.

The old front after rear method was based on the needs of single-circuit cars.

Pfc. Parts

That was it. The problem was I wasn't looking at the ABS procedure since I have an '85 without ABS. Thanks for the reminder. So the sequence isn't really the reverse of the "normal" protocol, it's passenger front, driver front, passenger rear, driver rear. Assuming you don't bleed the MC.

Thanks Mike.

PRSWILL

Mine goes on Tuesday for new fluid and a bleed- you guys scare me - I will let the pros handle it.

Pfc. Parts

You mentioned you replaced the lines with SS? Could you have a bad line on that wheel? Sometimes you get bad parts right out of the box. If no fluid moves at all I'd say that indicates some sort of blockage or a bad MC? Maybe both? You were using the pedal pump method before switching to the pressure method, perhaps a blocked line on the passenger's front blew the seals on your MC while you were pumping it?

vanster

I had good flow on the ft right earlier today. Maybe I did blow the MC...argh!
I only pushed the peddle after thought I had a successful bleed. It had some pressure then went to the floor again.
I'll try Dr Bob's method again.

Nicole

No idea. I have never seen it, but was told by Porsche employees that that's how it's done..

dr bob

"Normal" fluid flush and bleeding is a no-worries procedure. Challenges show up when you let all the fluid out and the system gets air in it. In the ABS cars, a little air passed into the ABS unit is a bit of a challenge to clear out. Lesson is to push new fluid in to replace the old stuff if you can, and avoid leaving the system open. Something as easy as a piece of Saran Wrap under the MC cap, and some caps over any exposed line ends will save a bucket of aggravation. It's all solvable with patience and persistence though. 928 DIY'ers should probably stock a small drum of that next to the toolbox.