Just putting my MCS on the road, most things recoed incl booster. was OK during tests but getting set for RWC, appled brake and they locked on, disconnect the vac line and block it, brakes work as if no booster. Any idea please?

I don't know if this will do anything to help you, but, is there a one way valve the vac line to the booster? If you put one one, make sure it's the correct way round, I broke my booster when my car missfired, (air shot into the booster through the vac line and did not leave) and yeah not good, they should be labelled 'SERVO' with an arrow pointing towards where your booster would be.
http://www.minispares.com/Product.aspx?pid=36307


I got mine from a Rover SD1 for nothing from a local wrecker.

_________________
1964 Morris 850, 1330 Supercharged - 81.8hp atws.
1975 Leyland Mini S 1100S powered - Nice and reliable.
1977 Leyland Mini LS - Project LS-T

I did the rebuild of the booster..... it has all new seals, diaphragm etc, and a new non-return valve... its the first one I've rebuilt, and has HJM says, it worked well for a little while, but now it's working too well

from talking with HJM on the phone (he's 400km away), its as if the brake pedal is being pushed whenever the engine is running, I got him to undo the outlet brake pipe on the booster and let a little bit of fluid out, which released the pressure, but as soon as he put his foot on the brake pedal again, they locked on again....

I don't think I had any parts left over when I reassembled it ...

Did you also rebuild the master cylinder or replace it to, disc brake cylinders and drum brake cylinders have a different fluid return valve in them and if you fit a drum barke kit to a disc brake setup this is what can happen, it can also happen if you have fluid contamination, never had it happen on a mini but have had it happen on a mazda after the master was rebuilt and they left a jelly like stuff in it which floated on top of the fluid in the reservoiur

Jon

_________________
Roundies and Clubbies(except vans) are both ugly that is why I have Midas and a van
1293 Mini Midas Bronze,1275 Clubman van

If he replaced the brake master was it replaced with a clutch master? Harder and harder peddle untill it locks up. How far is he getting before brakes lock up?

Steve

the master cylinder is a tall tank tin one which I had in my blue S, then in my van - it came out of the van and I screwed a bleeder into it where it stayed for 18 months or so, now its gone into HJM's car.... I didn't even clean it out my first reaction to the problem was a bit of muck working its way into the booster and blocking it somewhere, but I don't know...

the booster puts pressure into the system immediately after the engine is started, even without putting your foot on the brake pedal

just doesn't make any sense to me - I put the booster together, but I don't understand what makes it work, where the fluid goes, how the engine vacuum is supposed to increase the pressure only when the pedal is depressed

The little piston that pushes the vac valve is stuck. (I am guessing)

If you have the original blue covered Australian workshop manual (Leyland one) It has a full discription on how the booster works

_________________
David L


My greatest fear in life is that when I die my wife will sell my Mini and tools for the price I told her I paid for them!

Thanks Steve. It was OK for a few months, just moving it around on and off a trailer to the painters, then in and out of the shed, around the block. Then last night I backed it out of my garage and felt resistance, the brake lights were on and the pedal hard, Turn the engine off and had half a pedal. Blocked the vac hose and the brakes work like my Mini K !!

is that the one inside the breather that hangs off the side? underneath the smaller diaphragm? I did have trouble with that piston, the one in the kit was slightly different to the one that was in it

Wombat, I did follow the diagram in the big red book, but there were little inconsistencies ...

Yep, my guess is that the piston is not returning all the way and holding the vaccum valve open.

I Agree with morris 1100, This happened to me also when i rebuilt my booster, the little control valve is stuck in the "applied" possition, it cant be tight in the bore, make sure there is no pitting/ scoring ect, and it's smooth to slide up&down. did you lube the seals with brake fluid? [/quote]

_________________
(KILL #53) Help MotherEarth, squash a Bug.

Thanks for you help and interest guys.
Turns out the piston was OK but the small rubber cup in the end of the cap which the small diaphram presses on was not sitting centre and disslodging. The foam surround needed to be trimmed a little to allow the cup holder to sit in the centre. Works perfectly now.