mk 1 auto

[rewthalicia]

hi all, i just tried doin this message but it doesnt appear to hav come up so trying again.
ive got a mk1 gl auto 1983 1457cc
it failed its mot 2.5yrs ago and has been sat in the garage ever since.
the mot was done by a previous owner at a garage i dont know.
basically it failed on " brakes do not rise & fall at the same rate" and "rear brake compensating valve inoperative"
some people hav mentioned to me that they dont think this model has one of these valves anyway, as usually only the higher engined ones did.
wondered if any one has any ideas? ive checked the haynes manual but i know naff all about cars and the book may aswell be in a foreign language!!
cheers

[JFryauff]

Some Rabbit's of that era had "combination valves"...on the driver's side of the engine compartment, below the master cylinder on the fenderwell, you'll see a fairly large "T"-shaped block of aluminum with four brake lines, two in and two out and a single wire on top. This is the "combination valve", which regulates pressure to the rear drums, contains the residual valves, and has a left-right bias switch that illuminates the instrument cluster "brake" light if one half of the diagonal braking system fails. That's it.

Other Rabbits (most through 1979 and some 1980 and possibly 1981 diesel Rabbits and Rabbit Convertibles) have a "load sensing brake pressure regulator" at the front of the left rear suspension.

If you have the load sensing regulator, there are a few things to check and/or replace. Make sure the spring that goes from the pin on the rear suspension piece to the lever on the regulator itself is good. It's common for the spring to rust through and break. If it breaks, the regulator senses no load at all and will basically turn off the rear brakes entirely. I'd just replace that spring if you have that setup. There's also a pair of inline "residual pressure valves" in the rear lines that tend to fail and cause minor rear brake issues (usually a slightly longer pedal than you really want, but with good brake operation at the bottom). If your pedal has a lot of dead space at the top, it's worth changing out the residual pressure valves.

I'm very skeptical that any safety inspection would be able to diagnose a failed combination valve. How did they test the car? Did they just drive it over something and apply the brakes? Or did they connect brake pressure guages to a front and rear wheel? If it was any kind of driving test, there's no way they could tell if a VW combination valve is working properly from that. They could probably diagnose a broken spring on the load sensing regulator setup, but they probably can't diagnose a failed residual pressure valve or failed residual pressure section on the combination valve without guages hooked to the brakes.

I would suspect that your leaking wheel cylinder might be the cause. One thing to remember is that the brake system not only controls front to back, but also has 2 separate regions per say.

Front left; Rear right.
Front right; Rear Left.

This is so that is one region fails the other region will still work. One of these systems could be weak like at the master cylinder level. Personally I would just re-do the entire brake system, Master cylinder and wheel cylinders and do a brake flush. It probably needs it anyway and is not that expensive, atleast here in the US. I'd like to find out what kind of prosioning valve you have in the car?...and if it is the early load sensing in the rear or just the proportioning valve on/or near the master cylinder?

[rewthalicia]

hi &thanks for the reply.
they did a standard MOT test on the car. if u know this already then sorry. here cars hav to hav one every year to get insured and taxed. its basically a safety test, not really whether the engine is actually working as such!! u pass or fail on various points like
condition of service brake system
tyre condition
headlamps

well next to the fail they put that this compenating valve was inoperative. then the girl who had it sold it to me as itwas going to b expensive to repair.

ivejust looked in the manual for the golf, it lists the features of the braking system saying
" Brake pressure regulator*, loadsensitive"

the * means "items of equiptment marked with a * are only standard on certain models or are only available as options on certain models"

im gonna call the dealer & ask, make a prat opf myself as usual!!

[rewthalicia]

aparently it should be a load sensitive hydraulic one or whatever. he was confusing me & said to bring it down. At £60 per hr labour not a chance!!!! no wonder old VW hardly ever hav full VAG service history!!

[Fourt1thirty]

Originally Posted by JFryauff

Other Rabbits (most through 1979 and some 1980 and possibly 1981 diesel Rabbits and Rabbit Convertibles) have a "load sensing brake pressure regulator" at the front of the left rear suspension.

If you have the load sensing regulator, there are a few things to check and/or replace. Make sure the spring that goes from the pin on the rear suspension piece to the lever on the regulator itself is good. It's common for the spring to rust through and break. If it breaks, the regulator senses no load at all and will basically turn off the rear brakes entirely. I'd just replace that spring if you have that setup. There's also a pair of inline "residual pressure valves" in the rear lines that tend to fail and cause minor rear brake issues (usually a slightly longer pedal than you really want, but with good brake operation at the bottom). If your pedal has a lot of dead space at the top, it's worth changing out the residual pressure valves.

I've seen this posted in other places on the internet as well, and I found it in ETKA. What's odd is that on the tons of cars I've taken apart, and the 15 or so MK1s I've owned, I've never actually seen one. I need to go out in the yard and rummage through some cars until I find one and get to know this system better, it might be of some use to me.

Seems to be in german production cars with power brakes and early style master cylinders. My '78 German car has manual brakes which is why it didn't have one and the 16v scirocco and multiple cabriolets I've taken apart had later style master cylinders with prop valves up front. US production cars never had them... so somehow I've just managed to avoid this one part.