I decided to pull one of these out of my parts bin to see whats inside. I just couldn’t help myself with all the controversy here. Heres some pictures of what I found. PMB sells a rebuild kit with all the seals you will need.
Just a word of caution before you uncork the bottle and let the genie out. The 4 10mm bolts that hold the bracket on also hold the unit together. Thats a pretty strong spring so go slow and alternate which bolts you remove. Do them each a little at a time.
The nut with allen screw on the back of the black can should be left alone. It is used to adjust/change the pressure to the rear wheels.Click to view attachment Click to view attachment

You can count the number of turns or measure the exposed threads so when you take off the nut, you can reinstall to the proper spot.

My thoughts exactly,what is the rebuild kit ? thinking 2 piston 'o' rings and a washer for the cap nut...don't really know.
No mystery about these valves they are used in 70's and up VW Golf front drives,my 02 VW transporter has one but in front drive layout they are activated by the rear axle beam under braking load so the function is basically the same result.
VW and Porsche in this era were always equipped with good brakes.

Didn't @Eric_Shea have a writeup somewhere about the magickal inner workings of the valve?

Yes....and as is sometimes the case, spirited discussion prevailed and then ensued towards his direction.


I recall at somepoint it was proposed that Rube Goldberg invented it and then the "discussion" veered off towards somewhere between fluid mechanics and quantum mechanics

Kind of the opposite of Rube Goldberg in my book. Far too simple.


When I did mine, I didn't trust that the adjustment on a 50 year old spring would still be in spec. I reached out to Eric about sending it to him for adjustment. He said you can just adjust it to where there is 10mm of threads protruding from the spring housing. If Eric thinks that's good enough, that's good enough for me. The local Porsche guy said they road test them to make sure the rears don't lock before the fronts.

There is a test and adjustment procedure in the factory manual but it looked a bit complicated.

Yes...it is a simple device...mearly trying to characterize the thread back then. The controversy if I recall was that if it were actually needed and could replaced by just a tee.....oooops.


Eric is a great guy and I didn.t understand why he was getting speared in the thread. I talked to him personally about it onetime at the last WCR over a beer and we both thought that getting the brake line installed in it was one of the most difficult things to work on in 914. I thought installing the fresh air box with all the control cables was a very close second. I think I invented some brand new expletives to the English language when I restored mine.

The kit provides the needed parts and a tool that could be improved.
The units I've taken apart have pits in the cylinder walls.
Anyone have a remedy ? other than sleeving ?
Does a rebuild still work even with a pitted cyl. ?

I pretty nearly always adjust the spring pressure by installing a longer bolt and turning it in another 1/4" or so from the factory position.
That, and better pads, usually takes care of weak stopping performance.

Reading is fundamental...

Click to view attachment

My bad. I was just giving you a hard time and forgot to put one of these behind my smart ass remark.

I thought Eric said a “T” works better!!

You remember the thread! Low and behold another thread pops up with M calipers and a "tee" installed in place of the Rube Goldberg device....no offense taken......ever.

Porsche probably installed it to keep the lawyers happy. However, having the rears lockup first is a bad thing that should be avoided. Just wish they’d have installed a bleed screw on the assembly to make bleeding the rear brake lines easier.