Master cylinder is likely leaking, get a new one

Hey there, time traveller!
This article was published 12/09/2003 (8093 days ago), so information in it may no longer be current.

QUESTION: The brake fluid in my 1978 Ford car has been disappearing for the last couple months. The car has standard brakes (no power assist) and a dual chamber for the brake fluid in the master cylinder. I have looked for leaks but can’t find any. The fluid in the front part of the master cylinder seems to stay up but I have to keep adding fluid to the rear chamber. Is the fluid somehow evaporating?

ANSWER: Brake fluid does not evaporate at any appreciable rate, and the master cylinder is sealed with a cap, so the fluid must be leaking. On brake systems with dual reservoirs, each chamber in the master cylinder feeds two wheels. Knowing how the brake system plumbing is connected aids in diagnosis. You haven’t told me what model your Ford is so I don’t know how the brake plumbing is connected on your particular car but here are typical systems.

Most front-wheel drive vehicles use a diagonal split brake system where each chamber in the master cylinder feeds one front wheel and the opposite side rear wheel. This provides balanced braking during partial system failure. A fluid leak at rear drum brakes may not be spotted for quite a while unless the leak is large. Then a dirty wet stain will show at the bottom of the rear brake backing plate. Smaller leaks can be located by removing the rear drums and looking for stains. Look for wetness around the pushrods of the brake wheel cylinders. Any fluid here indicates a leaking wheel cylinder that should be replaced.

Most rear-wheel drive cars use a front/rear split system with the front chamber of the master cylinder supplying fluid to the rear wheels and the rear chamber supplying fluid to the front wheels. If it is the rear chamber of the master cylinder that is loosing fluid, it is probably leaking as it goes to the front wheels or from the master cylinder itself.

Master cylinder leaks often show up as peeled paint on the power booster assembly (Brake fluid strips paint quickly!). Your car does not have power brakes so if the master cylinder is leaking, it will leak into the interior of the car by the brake pedal and run down the firewall. Look under the dash for stains and peeled paint.

Replacing the master cylinder is a better bet than rebuilding it and costs just about the same. Be sure to bleed the brakes completely of any air using only new brake fluid before driving the car again.

QUESTION: I am driving a 1988 Chevrolet Caprice station wagon and sometimes have to carry a heavy load for a few hundred kilometres. The car has rear coil springs and sits quite low when loaded. I am debating whether to install air shocks to help support the extra load or to install new shocks with helper coils on them. I have been told either one will work but the air shocks take more time to install. What do you recommend?

ANSWER: I would install the air shocks. Here is why. You only carry heavy loads sometimes. The shocks with helper springs would give you a harsher ride when the car is unloaded. The air shock’s pressure can be adjusted to give you a normal ride when unloaded and added support when loaded. The extra work to install them is worth it.

The plastic airlines are the weakest link in an air shock system so route them carefully to avoid kinks, sharp edges, and locations where stones thrown from tires could damage them. When operating the car with air shocks, maintain a minimum of about 15 psi in the system to prevent the rubber air bladders on the shocks from being damaged and do not inflate it higher that 90 psi. Of course, follow the air shock manufacturer’s recommendations for best performance!

Jim Kerr is an experienced mechanic, instructor of automotive technology and freelance journalist. You can e-mail questions to Jim at the address below.

kerr.jim@sk.sympatico.ca