Front brakes locking up after system overhaul

M

miket

New Member
Having overhauled the front callipers with new seals, pistons, similar attention to the servo/slave cylinder and fitting of a new master cylinder I find that the front brakes lock up when I connect the vacuum hose to the servo. Without this the brakes release and wheels run free. I've stripped and checked all components three times now but to no avail. It seems like there is a build up of air pressure without it being vented. Have checked valve on slave cylinder and this is not stuck and appears to move freely. Last dismantle and further detailed inspection however I noticed 'pushrod' within slave cylinder had a tapered end unlike the flush end shown in repair manual. The one fitted to my single line system seems to resemble the type fitted to the dual system, again as shown in the repair manual. Could it be that my servo in a previous life has been built using an amalgam of different fittings and thus fails to work properly?
I'm at the end of my tether and would welcome any views on prognosis before biting the bullet and investing in a replacement servo.
Who offers best service for this?
Any help / avice would be greatly appreciated.
Mike
 
Hello Mike,

I have experienced exactly the same situation, the culprit is the air control valve within the booster slave cylinder. If you remove the air control valve cover (the white mushroom) you will see a coil spring as the cover comes away. This is a modification spring that was never part of the original design, subsequently being added later so as to assist the valve (or piston) if you prefer in returning. The valve or piston that I am speaking of is the tiny one in the top of the slave cylinder,...the probe on the air control diaphragm pushes against it. Below the piston there is a vanishingly small hole that allows brake fluid to enter and exit. This must be clear, else you will have problems like you have now, plus the air control piston must be free to move within its bore. Don't use rubber grease as it may well stick which will also give you the same result as you have now.

Best of luck, and do lets know how you go,
Ron.
 
Gave in and have sent Servo for professional refurb.
Awaiting return and hopefully resolve problem and get the car on the road for the summer.
Thanks for all the help and advice I received on this issue.
M
 
After fitting a refurbished brake servo unit , the front wheels are still locking up on starting engine. Experimented by by passing servo and same problem still persists although not as pronounced. Seems problem mighth now be master cylinder but this was bought new too( poss faulty? but stripped down and nothing untoward). I am reluctant to spend another circa £200 for another M/C but I'm at my wits end! Last thought is the PDWA? connection terminal but assume this is simply a connector with no valves etc that might be amiss.
This is the last issue before I get the car on the road but at this rate unclear what year this might be!
Any help greatly received.
Mike
 
I've had the same, only way to free of the brakes was to bleed them, it was the adjustment of the peddel height. No mention of it in the manual, Harvey saved me on that occasion.
 
richarduk said:
I've had the same, only way to free of the brakes was to bleed them, it was the adjustment of the peddel height. No mention of it in the manual, Harvey saved me on that occasion.
Click to expand...

Are you sure there's no mention of it? I'm fairly certain that's how I learnt that trick, from reading the 2000 workshop manual - proper Rover one, not Haynes. The height above the floor pan - carpet pulled back - is vital when replacing both the brake and clutch master cylinders.
 
The pedal height is in the book, all I would add is that the setting they give needs to be "optimised" a bit!
 
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