505 rear brake compensator

Does anyone know for sure how these things work? I'm asking because I originally thought my one was buggered since the piston inside it couldn't be pushed in and out easily, but I've just bought a new compensator, and its piston doesn't want to budge either.

Does the piston only move when the brakes are applied, the lever underneath the compensator limiting how far the piston moves out of the housing? This is my theory. Also, is there a quick test that can check if the compensator is working?

Thanks guys,
cheers!
Richard

How much was it?

I have booked my car into Graham Birrels for an inspection of the brakes tommorow, because the brakes chose to stop working as i was coming down a hill near my house...

I need a new compensator mainly because my one leaks (lots) of fluid.

The 505 rear brake compensator is a load sensing device.

As the all up weight of the vehicle increases (full fuel tank, boot luggage, passengers in the rear seat, trailer with down load on the tow ball etc), the load sensing arm on the rear suspension moves closer to the base of the compensator piston.

This has the effect of limiting the displacement of the piston downwards, thus ensuring a proportionately higher brake pressure distribution to the rear brakes.

A common mistake when bleeding the rear brakes (with the vehicle rear wheels off the ground), is to not have a dummy load applied to the compensator fulcrum arm. The dummy load simulates increased pressure distribution to the rear calipers, which improves the bleeding process.

Leaking compensators should be replaced, and as with drum brake wheel cylinders etc, you can gently peel back the piston dust cover to check for signs of fluid weeps. Make sure the cover is correctly replaced, otherwise moisture, dirt etc will hasten the demise of the compensator.

I have yet to replace a compensator on either the STI or the GTI, but have fitted a new master cylinder to each of them. In the case of the STI, it was because of the brake pedal travelling to the floor on the first application of a morning. With the GTI, it was because of fluid weeping between the master cylinder and the brake booster.

Caravelle Imports in Melbourne supplied the cylinders for less than $ioo.

Cheers,
Kim.

Richard,
I forgot to add that, with the rear of the vehicle jacked of the ground, application of the brakes will cause the compensator piston to descend towards the load sensing arm.

Otherwise, it is very difficult to manually move the piston.

Cheers,
Kim.

Thanks for the info Kim, that cleared up alot of confusion I was having! I too have replaced my master cylinder, because it was leaking where it joins the booster. When I bled the system, I put a spanner between the compensator and the fulcrum as the Haynes manual suggests, but didn't really know why I was doing it.

I'll jack up the car tomorrow and test it properly. Hopefully there's nothing wrong with my original one so I can get a refund for the one I bought, if they'll let me return it...

Jason, the compensator cost $62 at Regans (they quoted me $65.30 on the phone, don't know what happened). A nice black painted thing with "Bosch" stamped on the side, but it really pongs of the anti-corrosion stuff they treated it with.

I hope things aren't too serious with your brakes, didn't run into anything on the way down the hill I hope... eek! eek!

I'll post an update after my test tomorrow.

Thanks,
Richard

>UPDATE<

I put the car up on stands and borrowed my dad to stomp on the brake, but things didn't go quite as expected. When the brake was pressed, there was no movement of the piston, but when the brake was released, the piston would descend about 1 cm and return back up straight away. Very strange. We tried a number of times, but it would always behave the same way. It also didn't matter how hard we pressed, the piston would only emerge and retreat the instant we released the brake.

I looked at it closely but didn't see it leaking anywhere. Is it stuffed? If it is, is there any chance cleaning it out will fix it?

Richard

^BUMP^

frown

Don't ask me... all I ever knew how to do with them with any competence was watch them leak.

As some here may remember, a few months ago, i took my car to ABS brakes (in Fern tree gully) and they told me that aside from the leaking master cylinder, everthing was fine and that i should let them replace it for $400 roll_lau moon
ANYWAY, so i replaced it, and not much changed, except i was losing less fluid, and the internal carpets were less wet. While bleeding the system, i noticed the rear compensator was leaking a significant amount of fluid, out of it's rubber seal. (Surely ABS could have seen this)
So, this week i took it up to my friendlt local Pug mechanic who looked at it (for nothing) and tested everything and found that the LH front caliper and the RH rear caliper were both seized, and that the rear was permanently holding the rear wheel (which explains a few things), and that the compensator was leaking and inoperative. How cold ABS have not picked this up???

Anyway, so i have hunted down a good front and rear caliper ($50 and $100 respectively) and will replace them (very) soon.

Not Happy
moon