Residual pressure in rear brakes and no auto adjuster
Hi all,
My G is currently in a garage having had lots of work after a front wheel bearing failure. While there they have fitted new rear shoes (direct from MB) as the old ones were very worn. The system is holding a residual pressure, so the pads are rubbing. Any manuals I've found all refer to an automatic adjuster, which isn't present on mine. Has anyone seen this before, and do you have a solution?
For reference, I have a 1988 300 GD.
Many Thanks
Hagar
The pads do not have an auto adjuster.
There should/must be the pressure compensator valve in which is connected to the rear axle via a rod and is mounted above the rear axle on the chassis in a pretty well tucked up position. This valve should only change the amount of pressure the rear brakes see and it should not hold pressure on. Mine needed a fair bit of TLC to get it moving freely due to rust.
Holding pressure could be damaged flex hose because someone used a hose clamp on the hose.. a pet hate of mine! This ruptures the inside of the hose and can turn them into one way check vavles. Slacken fittings until the pressure goes away would be the easy diagnostics - well except getting any nut undone probably means a new brake pipe by now!.
I know not of any such fitted device near the front; please post a picture of it.
I know not of any such fitted device near the front; please post a picture of it.
It's a one way valve and it's in the near side front shock tower on the 4 way adapter
whats the purpose of that? if it prevents reverse flow then the brakes stay on or is there something missing?
Thank you Gordon,
I know the valve now.
Thank you Gordon,
I know the valve now.
Some cars have a brake failure switch where a piston sits between the 2 circuits of the master cylinder. If the pressures are different in the circuit it causes the piston to move over and that causes an electrical switch to close.. and a light turns on at the dash. No idea if the G has one .. but some cars do.. it has 4 pipes (in/out for each circuit) plus an electrical connector.
Is it one of these?
This valve is designed to maintain some pressure in the rear brake-circuit so that the pedal travel is minimised on the next application (Brake shoes have greater movement than pads on disc, especially if you don't have an auto-adjuster). It's important that you have it replaced. It's known as a Brake Residual Pressure Valve. (Not the proportioning valve connected to the rear axle)
Cheers guys,
A quick update, just got it back from the garage and brakes working fine. Load valve as checked and is all good.
There is a second valve near the front of the car, (I didn't do the work so not exactly sure where) which had a ball and spring in it. this was what was holding the pressure in the system, the ball was smaller than the spring so was just free to move around inside. the garage have removed these and they work fine now, Any ideas?
Thanks
Hagar
This valve is designed to maintain some pressure in the rear brake-circuit so that the pedal travel is minimised on the next application (Brake shoes have greater movement than pads on disc, especially if you don't have an auto-adjuster). It's important that you have it replaced. It's known as a Brake Residual Pressure Valve. (Not the proportioning valve connected to the rear axle)
Well put. I try to explain but people think i mean it keeps the pressure on the brake shoes and they are rubbing. Its explaining it easily thats difficult. To me its been on cars for years that i have repaired and maintained but if you are not 90% mechanically or hydraulically minded its confusing
Showing them face to face would be easy
I did not know the rear adjusters are automatic ones on the majority of g wagons
Ive removed and cleaned all mine and they have a chance of working
I thought they were manual like old 210 vans that were wind on or wind off
Very clever set up but age and off roading and salted roads upset the affair
Great thanks guys, I will get him to put them back in at the next service. the part that was strange for me was that the ball is smaller then the spring and therefore neither really did anything. I assume this is because someone else has messed with it at some point. Do you know what sizes these should be?
thanks
Hagar
I did not know the rear adjusters are automatic ones on the majority of g wagons
Ive removed and cleaned all mine and they have a chance of working
I thought they were manual like old 210 vans that were wind on or wind off
Very clever set up but age and off roading and salted roads upset the affair
Yes, quite clever, they work by ratcheting when you brake, going in reverse. The moving parts are stainless steel, but they do require cleaning every now and then.
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Cheers guys,
A quick update, just got it back from the garage and brakes working fine. Load valve as checked and is all good.
There is a second valve near the front of the car, (I didn't do the work so not exactly sure where) which had a ball and spring in it. this was what was holding the pressure in the system, the ball was smaller than the spring so was just free to move around inside. the garage have removed these and they work fine now, Any ideas?
Thanks
Hagar