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KDJones2000 · Joined: 14 Sep 2010 Posts: 322 Location: Phoenix, AZ

We have tried several times now to fix up our brakes on this (soon to be) crapcan race car of ours, and now it seems it is time to ask the knowledge base for answers.

What has been done:
1) Front calipers rebuilt (Thank you Zackhausen Racing) and reinstalled
2) Rear wheel cylinders rebuilt
3) Original master cylinder cleaned up and reassembled. Very poor pedal, so master cylinder replaced with new ATE unit (Original was also ATE).
4) Rear brake shoes adjusted to be just scraping the drums.
5) Bled all brakes using BOTH pedal and Motive Pressure Bleeder at 15psi.

Some notes:
a) The original master cylinder and reservoir had a bunch of fine black stuff in it, which was carefully and as completely as possible removed. The reservoir was for sure clean when switched onto the new master cylinder.
b) Even with the Motive using 15psi on the fluid reservoir, no fluid comes out of the rear bleeders. The only way to get them to bleed is to press on the pedal, even with the Motive in place.
c) We have tried bleeding using the cross technique and the more normal farthest to nearest order. Same results no matter what.

Results: Very soft pedal. The first stroke of the brake pedal and it goes all of the way to the floor. If you pump the brakes 4-5 times, you get a "sort of" decent pedal, but it is still spongy. When we left the 15psi pressure on the fluid reservoir, then we get a "sort of" decent pedal on the first stroke. The pedal may be slowly going down when held after pumping, but it is hard to tell with it so soft.

HELP!

About the only recommendation I know of that we haven't tried is to bench bleed the master cylinder.

Everything in my experience is pointing to a second bad master cylinder, but before we spend the dosh for another one, I wanted to see what you guys thought....

Thanks, Keith
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1987 924S SPEC car
Team Toofah Racing
1988 944 Turbo S

924RACR · Posted: Fri Dec 10, 2010 1:21 am Post subject:

You haven't replaced the hoses though, have you?

I've never found pressure bleeds to work all that well; nothing seems to work better than the old foot-bleed.

You may even still have a blockage going to the rear, possibly in one of the steel lines. Do you have access to shop air? Hitting it with an air gun may clear it out, or at least verify the blockage location...
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KDJones2000 · Joined: 14 Sep 2010 Posts: 322 Location: Phoenix, AZ

Oops forgot to mention that the rubber lines were also replaced. You know, that brings up another possible item to check - maybe there's some trapped air in the two front caliper hoses? Hmmm. I would have thought that the Motive should have taken care of any bubbles.

We do have access to compressed air, but just not sure where to break the system to clean out the back lines? That is a good suggestion, one that we thought of but have not yet done. We do have clean fluid going to the rears when bleeding by pedal, and after you pump the pedal all 4 corners are actuating (Can't turn the wheels by hand).

The main issue is that the first pump of the pedal is going to the floor. I had searched on the forum here and the only thing I could come up with is that you can experience this when the rear shoes are out of adjustment. We have done that adjustment to make sure that wasn't the issue.

Thanks for the input!
_________________
1987 924S SPEC car
Team Toofah Racing
1988 944 Turbo S

Grenadiers · Posted: Fri Dec 10, 2010 2:52 am Post subject:

I have found that my Mity-vac 'pulling' the air out at each corner works real well if it's just me and the car. Second best for me, is having someone stomp on the pedal about 5 times then hold it to the floor while I open the bleeder. Do that about 5 times on each corner removes the air as well. Bench bleeding the master before install is just about mandatory.
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BionicBalls · Joined: 05 Jan 2009 Posts: 642 Location: Charlotte, NC

Yeah, the foot method works best for me. It can take up to 10-15 times though. I had them pump until i saw the new fluid coming out (slightly lighter in color)
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1980 924 NA
1982 931

KDJones2000 · Joined: 14 Sep 2010 Posts: 322 Location: Phoenix, AZ

RC · Joined: 25 Mar 2007 Posts: 2637 Location: Australia

Check the adjusted length of the pedal pushrod to booster. there should be a little slack. Often an over zealous but unknowledgeable person will adjust it too far in an attempt to compensate for pedal travel or an insufficiently bled system. This stops the MC piston from returning fully and obstructs the inlet passage from the reservoir, hence little or no flow. IIRC there is no adjustment on these booster push rods, but if do you recall seeing one, then check it is not too long and the MC sits flush with no preloading.

Unless you have a multistage compressor capable of >1000 PSI in your shop, an average weight Joe with his boot on the pedal will easily create far more pressure than any air line to blow out any crud. Air is an easy test though if both ends are open.
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924RACR · Posted: Fri Dec 10, 2010 4:43 am Post subject:

RC - good point on the pedal adjustment!

The advantage of shop air vs. pedal - though you're 100% right about the pressures - is that you'd be flowing a lot of air through, in volume, to push out potential obstructions. Maybe.

The foot apply won't really move those, because you won't have so much flow, despite high pressures.

It's worked well with fuel lines, at any rate... YMMV!
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
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BionicBalls · Joined: 05 Jan 2009 Posts: 642 Location: Charlotte, NC

When I did my 5 lug conversion, I didn't bench bleed my master cylinder and it took FOREVER to get proper fluid flow from the right rear. I was using a bleeder tool as well and eventually finished up with foot pumping. But it seriously took like 30 minutes to get good fluid flow back there.
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1980 924 NA
1982 931

datatrain · Posted: Fri Dec 10, 2010 8:15 am Post subject:

Just did rear cylinders. I use an old method with a short hose and a bottle. I open the valve, push the hose on the valve, put the other end of hose in the bottle, jump in the car and pump the pedal until I get an ounce or 2 of fluid in the jar. Then top up the MC and do it again. After maybe 3 rounds of this I close the valve and try the brakes.

As long as the hose is in the fluid no air can get into the system. It only takes one person and it works well for me.

I drove it for a day and did it again and repeated on a third day. Brakes are good now.

It'll take some serious pedal pumping along the way and a pint of fluid.
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ole billy · Joined: 03 Feb 2006 Posts: 49 Location: Lafayette, CA

Go back to RC's comments about rod adjustment. If the MC piston is not returning completely, no fluid is getting into the cylinder for it to push against. Had same problem with a 944 that PO had adjusted the rod out. Changed two MC and finally found the problem. I have always needed to bench bleed the MC. If you can attach short lines to MC and have them go into the MC resivor it will tell you if fluid is being pushed out. I used 6" lines and bend them with pieces of vacume lines into the MC res. You will be able to see fluid moving out of each line. If not the rod is not adjusted.

Good luck

KDJones2000 · Joined: 14 Sep 2010 Posts: 322 Location: Phoenix, AZ

Well, I am getting the rebuilt MC later on today. I will bench bleed this sucker and put it in the car Saturday.

Then we shall see how things go

Thanks for the inputs, guys!
_________________
1987 924S SPEC car
Team Toofah Racing
1988 944 Turbo S

Scorpio · Posted: Fri Dec 10, 2010 11:13 am Post subject:

YouTube taught me that a little grease around the bleed valve when bleeding the brake can prevent air entering through the threads. My moneys on improperly adjusted master cylinder rod. What's the method for adjusting correct travel ?
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KDJones2000 · Joined: 14 Sep 2010 Posts: 322 Location: Phoenix, AZ

From what I understand there is no adjustment for these vacuum assists. That must be why the manual states that you need to match vendors for the MC and Booster.

I will find out more on Saturday...

I picked up the rebuilt MC, and it has all of the hoses and fittings to bench bleed the sucker. We didn't have those (I didn't think that it would be necessary to do since we had the pressure bleeder) for our first attempt.

So I am hoping that it is some stupid air in the MC that we can't get out.
_________________
1987 924S SPEC car
Team Toofah Racing
1988 944 Turbo S

MRPETE · Joined: 07 Nov 2010 Posts: 67 Location: CALGARY AB CANADA

Sounds like you've got air in the syetem.
This can drive ya' nuts
You've done everthing perfectly but shit.
Air will hide in fhe High places,Ya gotta get aggresive and get it ALL out.
One little bubble will manifest into a MAJOR PITA
That little bit of air is in there somewhere
I know how you feel BUT just do it several times I thought my MC was FUBAR but a week later I got a bubble fart and all is well ever since Go figure?

Pete

PS Make sure all fittings are tight .It sounds stupid but air can get in(thin)but fluid(Thick)doesn't APPEAR to come out P
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