964 bomb disposal

[Paperboy]

Hi. New to the forum as a contributor. My 1992 C4 had a completed brake fluid change about 3 years ago when a hydraulic cylinder in the 4WD rear diff was changed. Realising it was about time to do another one I thought I would get stuck in, despite never having done it on the Porsche myself before. I thought it might also improve the more and more tiresome 40 second pump time in the morning, making my 964 far from an ideal getaway car.
All went well thanks to a lot of good prevoius threads on that topic and Adrian Streathers 911 Bible until it came to the accumulator. The bleed nipple just wouldn't budge. ( The one on the dual solonoid valve was rediculously tight as well.) After several days of rust penetrating oil , I considered a bit of heating, which was done very carefully considering were the plastic petrol tank is. ( I used a gas stove lighter, which is basically a large cigarette lighter). Finally, a bit of movem- No, a broke bleed screw.
Options? The car was drivable so book in to my local indie or porsche centre or having a go myslelf ? you guessed it.
On to Ebay and checking prices. New and used ones avaible. Opted for a used, tested one which could be deliverted the next day for under £70.00 including postage. ( What, fitting used brake parts? More on that later.)

So here follows the procedure used for the " bomb " change.

Before removing the bomb de-pressurise the system by pressing the brake several times until the pedal feels hard.

Disconnect and remove the battery.

Remove the cover over the ABS unit, held in place with one srew with a plastic top. Same cover as removed for an accumulastor bleed.

Placed someting absorbant under the bomb. Detach the three hydrauliclic lines. They are an 11 mm nut fit. Mine came of with and open wrench without any head shredding, but if you have an open circular brake line wrench in the same size all the better. The brake fluid leak from this step is minimal when the system is depressured. Clean the accumulator and hydraulic pipes/fitting as carfully as possible before this removal.


The accumulator is fitted as part of the ABS-unit right next to the battery. The whole unit is held togetter with a two part bracket system.

Also, the bomb itself has a bracket welded to the " back" of it which in turn has two bolts welded to it. ( see picture)

These bolts attach with M10 nuts to the smaller part of the ABS unit bracket through two rubber bushes. This give the accumulator a fair bit of movement even when " tightly" in place. This is quite obvious when doing the bleed. I assume one reason for this arrangement is to reduce any tension in the three solid hydraulic lines attached to it. ( Speculation: I do not know if brake fluid can "froth" with vibration? This set up would reduce this. )

The M10 nuts mention above are almost impossible to reach due to their position.
The procedure to get the bomb of the car involves removing it together with the smaller bracket section. Four M10 nuts are attached to 4 fixed bolts. 3 are visable in the second picture posted. One vertical on the floor of the car, one horisotal half way up the bracket and one vertical visible below and inbetween the hydraulic lines in back of the acumulator. The two first are removable with an M10 wrench/ socket. For the third one use a M10 socket on a 6-8 inch extension.

With these three bolts removed the bomb together with the smaller bracket is now loose. However it can not be removed due to the different directions of the three bolts, the horisontal one blocking any vertical movement.
To solve this remove the fourth nut which is positioned just to side of were the removed battery would be , under the ABS unit. see picture. With this nut completely removed from its floor-fixed bolt the whole unit can now gently be tilted and lifted enough for the vertical bolts to clear their holes and bracket together with the accumulator can be removed sideways. ( Makes complete sense when you do it).

With the accumulator out of the car the bracket can now simply be moved to the " new" accumlulator, see picture . The nuts holding this bracket in were fairly tight, possible slightly staked ? I reused the old nuts with no problem but as a precaution used a bit of thread lock. You do not want these working loose sínce they are almost impossible to inspect and re-tighted once things are fittted back on the car.

Refitting is pretty much a reverse procedure.

This of course has to be followed by a bleed. (I bled the acumulator 3 times and the the dual solonoid twice )

The end result? This was done two weeks ago. After first initial start up warning light and any leaks were monitored. All ok. A careful drive down the street and back with another leak check. All Ok. Slightly longer drive with engaging 4WD diff lock. Worked fine. After driving the car for two weeks- no problems. Pump start up time is now about 10 second after car stationary for 36 hours. ( Yess!) I know 30 second pump times are considered normal but my thought is the shorter the less long term pump wear?
The full brake bleed showed fairly clear fluids except the clutch cylinder which required about 300 cc before clear( seems like common occurance) . The diff was bleed with the poor man's Pete Seeger " If I had a hammer" technique, but only the lateral one which is reachable with the right rear tire off rather than under the car. Have to do the longitudinal one when I can rope in some assistance.

The "used" brake accumulator? Seems to be working fine, possibly even better than the original one since I can not remember the pump start up time be this short after the last complete brake fluid change/ bleed was done by a trusted indie, but my memory could be wrong. Any problems or leaks would be quite obvious on a system operating with the pressure the 964 system works at. Having owned three Citroens with hydraulic suspension and having worked on them might have helped me to take the mad decision to dive in with this job.
( The British Haynes Manual for the Citroen Xm grades hydraulic jobs as 2-3 Wrench jobs on their 1- 5 Wrench scale , a 3 being "suitable for competent DIY mechanics" . The 964 is my only car at the moment but I wouldn't mind a nice late DS next to it on the drive.)
PS. Tried to remove the broken bleed screw once out of the car but not possible even with careful drilling, so unfortunatly old accumulator definatly scrap.














Next project: during the brake bleed unfortunatly spotted torn inner drive shaft boot in the rear.

[Alan Goettel]

I think mine runs for about a minute, but it has been like that since I got it 2.5 years ago. I also have an RS America, but the C4 is no less fun to drive. Its a blast in the snow, when it all works its sure a cool system.

Thanks for the nice write up. Its in my near future.

[Beezztie]

I am about to do the same, replacing the bomb, so very helpful information in this topic. One question, regarding the bleeding, do you have to bleed the entire chain, so brakes, diffs, accumulator and valve block of only accumulator and valve block?

Reinier