ATE master cylinder issue?

[john walker's workshop]

made in czech republic now, not that that really matters. there's been a couple of cars lately with a fresh MC, which after a thorough bleeding, PMB rebuilt calipers and new hoses, still had a soft pedal at the top 1/2 of the travel. basically the rear brakes are not getting enough pressure. the fronts lock up on a hard stop. 80 and 82 SCs. all the dual stage MCs have a kind of hard "stop" that you can feel if you press hard enough, which is just the front system. usually you don't get that far because the brakes respond well way before you reach that point. the pedal on these cars is rather "soft" at the top 1/2 of travel. both cars got another MC after the first one felt weird, but the problem remains. lots of bleeding by pressure, foot and vacuum to no avail. not an air bubble to be seen. tried pumping out one piston at a time, prying it in just enough to slip the pad in, tried some road time over a few days and a rebleed. running out of ideas. everything points to a bad batch of MCs. anyone else had this problem lately?

[john walker's workshop]

bump for the evening crowd.

[Shuie]

Not sure if mine is bad, but I have the same issue in my 914 right now. Brand new 19mm MC, new lines, PMB calipers, new everything. I've done every brake bleeding technique that I know or have read about and I absolutely cannot get a decent pedal or any more air out of the system.

[manbridge 74]

Do you have a pic of said MC for comparison purposes?

Also, does the rod going into the booster (or the linkage going from pedal up into frunk) need a length adjustment due to different MC?

[db_cooper]

My experience is the czech made stuff..hydraulics, master cylinders, bearings and hard parts are of very good quality. Maybe better than the Germans at the moment.

A question..what was wrong with the original M/C that was removed? Leaking?

It sounds like the M/C internal piston for the rear brakes is stuck..could be poor quality..

I would take the M/C off and take it apart and see...sounds like the czech M/C is going to be replaced anyways.

[gamin]

Yes. Local very experienced Porsche only mechanic has had this very problem on a couple of cars. Very frustrating. Like you, tried all types of bleeding and other new mc's with poor results. This is a problem that needs resolution for safety reasons. We all need properly functioning braking systems.

[john walker's workshop]

both cars got another MC after the first new one didn't perform. the originals were just old and bleeding down under pressure like old MCs do. can't keep replacing them for nothing. possibly a rebuild kit in the new MC might help if i can find one. pelican and worldpac don't list one.

[pdtweeks]

John,

were you getting any symptoms discussed here ?
http://forums.pelicanparts.com/porsche-911-technical-forum/792049-brake-fluid-dribble.html

[Laneco]

Subscribing to this thread as we've got an 80 and an 81. If JW can't get the ATE replacements to work then there is zero possibility that I ever will when mine eventually fail...

angela

[Bill Verburg]

Originally Posted by john walker's workshop

made in czech republic now, not that that really matters. there's been a couple of cars lately with a fresh MC, which after a thorough bleeding, PMB rebuilt calipers and new hoses, still had a soft pedal at the top 1/2 of the travel. basically the rear brakes are not getting enough pressure. the fronts lock up on a hard stop. 80 and 82 SCs. all the dual stage MCs have a kind of hard "stop" that you can feel if you press hard enough, which is just the front system. usually you don't get that far because the brakes respond well way before you reach that point. the pedal on these cars is rather "soft" at the top 1/2 of travel. both cars got another MC after the first one felt weird, but the problem remains. lots of bleeding by pressure, foot and vacuum to no avail. not an air bubble to be seen. tried pumping out one piston at a time, prying it in just enough to slip the pad in, tried some road time over a few days and a rebleed. running out of ideas. everything points to a bad batch of MCs. anyone else had this problem lately?

It sounds like air in the m/c or possible a leaking seal on the primary piston

these are simple systems
the pedal pushes a rod which mechanically pushes the primary piston, in a nominally functioning system hydraulic pressure in the primary(front brake) chamber pushes the secondary pistons pressurizing the secondary(rear brake) chamber, springs and hyd back pressure return the system to neutral

if there is air in the primary chamber it can allow the primary piston to compress the spring between the primary and secondary piston causing the two to connect mechanically rather then hydraulically

Just a guess here

[db_cooper]

Looks like a rebuild kit is just not offered for the 911...not surprised..not many rebuild M/Cs anymore.

I would take the suspect M/C off and take it apart...depth of the piston compared with the old M/C prior to taking them apart..look at the bore and see if it is machined smooth..the pistons..are they clean with no machining debris? Seals intact? Not folded or poorly sized to the bore? Springs in there too?

Disassemble the old M/C and you took off and see if it is an application problem..compare...bores the same size? Springs? Seals?

The fault should be obvious..back when I did Porsche work..we did rebuild M/Cs with no issues..now with our litigious society..no way would I do that on a customers car (I would experiment on my own 911 and see what is going on with the czech M/C though) .....make a warranty claim with your M/C supplier once you know what is wrong with the czech M/C.

[db_cooper]

Oh, yes..and the ports drilled in the M/C..maybe the czech is missing one or not drilled out correctly..that would leave air in the M/C too.

[john walker's workshop]

i wish i still had the originals to compare.

bill, why would any air not bleed out after gallons of fluid? i even bled the brake light switches for that potential last tiny bubble.

[manbridge 74]

Have you tried bleeding the brakes while the car is running? Works on some makes with stubborn bleed issues.

[70basket]

I have been dealing with a similar issue on my Bug. I installed a front disc conversion kit on it at the beginning of the month and it has been taking up all my free time since. Just last night I was able to finally get a firm high pedal. After two masters, new lines, new rear cylinders and pushing/pulling 3 quarts of brake fluid thru the brake system it seems to have been air trapped in the driver’s side caliper. I removed the caliper clamped it and held it up at different angles while bleeding. When I heard my wife (whom I dragged out to the garage again) say the pedal felt good I could not believe it. Next up is my 911 and after this experience I am a little reluctant. The Bug has a flex line direct to the caliper so it was easy to move around when removed. The solid lines on the 911 will make moving the caliper around more difficult.

[safe]

What if you plug the flexlines, if it's still soft it must be the master cylinder??

[sig_a]

Bench bleed of M/C works sorta half assed, sometimes. Worth the effort if only to eliminate a potential source of trapped air. Another place air likes to trap is at the highpoint of any part of the brake line circuit. If flushing without adequate fluid flow rate and pressure, trapped air just stays in place at the high spot in the line. Repeated flushing at higher (15 psi) pressure pushing more fluid through needs to force the air pocket to move along with the fluid stream into and thru the caliper tap. I think the flushed fluid should almost squirt out in a stream. Then again, even this doesn't always make a rock hard petal, but in my experience it does seem to improve with each repeated flush. Days and days of driving the car also seems to somehow stiffen the petal. In my case, I also wonder if air could be somehow introduced from G50 hydraulic clutch.

Once the M/C is mounted, I buy cheap liter containers of valvoline or similar then power flush it thru repeatedly. I also use a rubber hammer and bang moderately hard on each caliper while the fluid is flowing hoping to free up any trapped air.

Hope this helps.

[Reiver]

All good tips but JW is a pro and I'll bet he's tried the tricks. I just bled my system after rebuilding my front calipers and replacing the hoses....stock MC...no problem...worked on the second go round. Nice pedal.
Shouldn't be this big a deal unless there is some other issue.

[steely]

Originally Posted by Laneco

"...If JW can't get the ATE replacements to work then there is zero possibility that I ever will when mine eventually fail...

angela

amen

[Trackrash]

I had a spongy pedal after I replaced my front calipers, MC, and the hoses. I fought this for a year. I finally replaced my new rubber brake hoses with braided stainless. Problem fixed.