Dash01

Can it be rebuilt or refurbished?

New ones are grossly overpriced, so is there a DIY hack? I ask because based on various YouTube videos of brake proportioning valves, these things are pretty simple devices with a plunger/piston and some seals of the same general description and materials as those found in a standard master cylinder rebuild kit. Such MC kits typically have a few O-rings, a seal or two, and a rubber boot, etc. and sell for a few bucks. Googling for brake proportioning valves to fit a non-ABS '86 951, it seems some vendors are asking over $1,000 for the complete part, but what about a rebuild kit?

Has anybody here looked into this as a vastly cheaper alternative to buying the whole new part?

Oddjob

Wow, had no idea those were now a problem to source.

I took one apart some years ago when they used to be under $100 for new from Porsche. I'll look to see if I have pics of the guts.

Your description of what is inside is accurate.

What's wrong with the one you have?

Dash01

I'm not sure that anything is wrong with my non-ABS brake proportioning valve, which is not leaking. Rather, my '86 951 is not getting the vacuum assist that it probably should, so I consulted various posts on this website as to probable causes, and found the proportioning valve to be a likely suspect, which other posters have fixed with buying a new one. Wondering what a new one might cost, I found that some vendors are charging over $1,000, whereas others charge ~$300 for the exact same Porsche part, yet generic adjustable aftermarket valves may sell $50-60 each.

This thing is a little piece of brass with a bore hole, seals, O-rings, and spring-loaded piston inside. It uses the exact same materials is a typical brake master or slave cylinder, i.e., DoT-3 brake fluid-proof plastic or rubber seals and metal impervious to brake fluid corrosion. Such "rubber" seals and boot probably cost a few cents each to make in a machine that spits out thousands of units per hour. The brass should last forever if reasonably clean.

This thing is also a generic VW/Audi part, mated to a VW/Audi vacuum assist, so perhaps it uses standard generic VW/Audi O-rings, cups, spring, boot, seals, etc. Various YouTube videos show how to rebuild brake proportioning valves of various other cars, not a difficult procedure IF such O-rings can be properly identified by size, material, etc.

So, does anybody here know what size O-rings, seals, etc. this unit has, and where good quality replacements can be bought?

Otherwise, a failed unit may be essentially good, but gummed up with waxy crud from coagulated brake fluid, humidity (water from the atmosphere as brake fluid is hydroscopic). This happened to my BMW motorcycle brake master cylinder bore and seals. The BMW dealer insisted 'pay up, sucker, they all do it' and wanted hundred of dollars for a whole new unit, rather than just sell a rebuild kit. I balked, not wanting to be price-gouged. So, I disassembled the unit while still on the bike, polished the bore with a Dremel tool felt pad and some toothpaste with mild abrasive, scraped the waxy crud off the (otherwise intact after many years of use) seals, and re-assembled. That was about 15 years ago, and after that cleaning she's still good to go. Maybe the same procedure would work with a Porsche brake proportioning valve (?)

Thoughts?

Oddjob

Oddjob

I don't know the connection between the prop valve and a vac assist issue. The prop valve, if it failed or leaked internally would either allow too much line pressure to the rear brakes (causing rear lockup before front) or the opposite, no line pressure to the rear brakes.

Not sure when the price on these jumped, and if recent, unlikely there is a rebuild kit available yet. But, yes the wear parts appear to be just o-rings, and hopefully they are standard size so can be relatively easily sourced.