My Scenic has just failed my MOT retest for having uneven rear brake pressure. I had the rear wheel cylinders changed for the retest but the problem was just as bad. Does anyone know if there is something else it could be?
The brake pedal is firm and the brakes feel ok. Is there a brake pressure regulator that could need replacing?
The car is a 98 Megane Scenic Phase 1 1.6e with ABS.
To be honest,the rear brakes have got to be miles out of balance to fail an MOT,to the point that one isn't working at all or one is grabbing on.
Did the rear cylinders leak on to the shoes at all before you changed them?.
Are they adjusted up evenly either side?
The brake limiting valve controls both rear brakes so doen't cause an imbalance,and as your car has ABS then I strongly suspect it won't have a valve anyway.
Hi Andy - any garage that changes the cylinders and not the shoes ain't competent in my opinion - considering they only cost around £15. Generally adjustment doesn't affect imbalance in the Northern Ireland test. As already said if the original cylinders were leaking then there is the distinct possibility either the shoes or drums have become contaminated. If that is the case the shoes will need replaced and the drums need to be cleaned using heat as the metal absorbs the brake fluid and will re-emerge when hot. As I live in Northern Ireland I have seen this type of MOT failure on many occasions.
You are right about the garage. When I drove the car after the wheel cylinder work, the brake warning light came on. They didn't even top the fluid level up.
Hi Andy - I recommend you replace the brake shoes and ensure the brake components are thoroughly cleaned. I use a can of brake cleaner. Decontaminating the brake drums is a bit more involved though.
1. Ensure all rusty edges and rust flakes are removed - I use a small angle grinder to remove the out wear lip.
2. Clean the brake drum thoroughly with brake cleaner.
3.. Set the brake drum up on something non-flammable ( I use a brick or axle stand.)
3. With a gas blow lamp at a low setting I start to heat the drum form the outside - constantly moving the flame around.
5.. After a while you will start to see any moisture or absorbed brake fluid coming to the surface and a light film of rust forming on the surface - this can easily be romoved with emery when the drum has cooled.
6. Do not heat the drum from within as there is risk of damaging the wheel bearing.
7. Afetr all component shave been correctly assembled fit the brake drum and operate the handbrake a few times - this will help centralise the shoes.
8. Bleed both rear brakes - until no air or bubbles emerge.
9. Using a screwdriver turn the knurled nut adjuster up a few notches at a time - this will involve nunmerous attempts- until the drum can just be fitted.
As a matter of interest what part of Northern Ireland do you come from - I live in the south east of Belfast.
Thanks for taking the time to write all that up. It's really helpful. I'll have to set aside plenty of time to tackle this but your instructions are excellent.