Insufficient rear brake pressure.

[jdw66]

Car failed mot today on rear brakes and emissions. Rear brake pressures were 13 and 15 respectively . In other words non existent. Anyway took the car to my mechanic mate and after much brake bleeding and adjustment to the shoes we only managed to get the readings up to 55 and 60. Obviously an improvement but not good enough. He's' now convinced the brake regulators on the master cylinder are not working, therefore there's not enough pressure going to the back brakes. Personally i'm not convinced. Any ideas would be appreciated. thanks in advance, John.

[steve bain]

Doubt they would both fail at the same time. Master cylinder could have popped a seal. Depends how the circuits are plumbed in.

[steve bain]

Also. You may have dented both rear pipes as they run next to each other and squashed them preventing fluid moving. In fact I would start there....

[jdw66]

Thanks. Sounds possible . I did change the rear beam last week (with help) , and both rear brake pipes were changed at the same time but it's possible the long brake pipes runing along the floor plan connected to the hoses could of been squashed.

[unknownmale]

Have you had the brakes apart?, the smallest bit of oil or grease on the shoes would kill them. Leaky slaves would also do this.

[jdw66]

One brake cylinder had to be changed due very slight leak. No contamination to shoes. Brakes shoes, drums, plus springs which are have only done about 3000 miles were transferred to a newish rear beam last week. Everything is basically new and spotless. Had the car on the lift today in the mot station again and found that one of the main brake pipes on the passenger side is slightly squashed , that could explain the problem with one side but not the other.
My mechanic friend (sort of ) is adamant the problem is the brake pressure regulator valves under the master servo, therefore he's not interested in changing any pipes. In fact we've fallen out of the matter because being the font of all knowledge he's certain he's right and i disagree. So i'm going to get somebody else to to do the job and i pray it works.
Another thing i noticed was the brake hoses were rock hard. Is this normal?.

[steve bain]

Yeah. They go brittle and hard after years! Which is fine but they can split easily. If you've had the beam off it does sort of seem like a good place to look too.

[james butler]

you could have like i did the wrong slave cylinders in there aswell.
theres a 14.5mm and a 17mm one
the bigger one will give you more braking effort which is why there fitted with brake pressure regulator valves and the 14.5 mm arent

[jdw66]

Thanks James. will take a look.

[Mark H]

I changed to 17mm Rear Cylinders from 14mm. Was able to do a comparison of the brake efficiency at a friends MOT station, seen a 15-20% increase in the rears (nothing you can actually feel), in addition the rears do not lock up.

The next step really is to go the whole hog and fit adjustable proportional valves direct to the Master Cylinder.

VW set it up originally that way, I dont think a bloke in his garge at home can really question there reasoning.....