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QI’ve overhauled my BMW 1502’s brakes. The twincircuit master cylinder is an unusual type, so was rebuilt with an assortment of seals. The servo is a known good secondhand unit. During the test-drive all four wheels progressively locked solid. I loosened the nuts holding the master cylinder to the servo and the brakes released. Test-drive number two with the master cylinder spaced out (the pushrod’s not adjustable) and the same thing happened. This time they wouldn’t release and I had to wait for them to cool down. What do you think? Dave Andrews, Amersham
AWhen all four brakes progressively lock, it’s because the fluid has heated up and expended. This pushes the brakes on, which heats and expands the fluid further. Normally, though, the fluid should be able to push up into the master cylinder and escape into the reservoir whenever the
brakes are off – so the master cylinder is the prime suspect. Some have a tiny compensating hole drilled just ahead of the rest position of the main seal. If the hole is blocked due to toolong a pushrod or by a slightly oversize seal, this will cause the problem you describe. And do make absolutely sure there’s a little play between the pushrod and piston when the cylinder’s bolted to the servo.
Modern designs sometimes have an arrangement built-in to the piston. This is likely to rely on a bit of play or free space somewhere in the piston assembly with the brakes off – and again an oversize seal may interfere with this. Try to find a correct full kit for the cylinder. Find a diagram, too, as some small mistake in reassembly could also be to blame. A fault in the furthest chamber of the twin-circuit master cylinder will likely only affect the brakes on that circuit (usually the rears), but a fault in the front chamber will affect both circuits.
A faulty servo can be tested by placing some pressure on the pedal and turning off the engine. If the pedal immediately reacts by rising and becoming harder, the vacuum seals or the oneway valve are defective. It should take some time or a number of pedal presses to exhaust the vacuum. Press the pedal several times, then hold it down and restart the engine. It should immediately react, usually by sinking. If not, this again implies a leaking vacuum hose or a defect. Vacuum leaks will lead to lean running, too.
Beware of ageing flexible hoses, used or new old stock. They can close up internally and act as a one-way valve. They can ‘set’ in position, fully blocking flow if flexed to a new position. Hoses that have gone funny can cause the brakes to stick partlyon or partly-off, or to hesitate before coming on. They’ll also make the pedal feel soft if they expand under pressure.
Make sure the calipers aren’t binding due to sticky pistons,
expanded seals or seized sliding elements. Check that drum brakes are adjusted correctly or that their automatic adjusters are functioning as they should – and also check the handbrake adjustment. Patchy oil, grease or brake fluid contamination of the linings makes the shoes sticky, which can lead to laziness or hyperactivity.
Many classics have an often-neglected compensating valve fitted to the brake circuit(s) of the rear axle, which restricts the pressure to prevent the rear wheels locking when the car’s lightly loaded. If it’s seized or incorrectly adjusted, the rear brakes might be doing more of their share of the work than intended and getting hot as a consequence.