brake system air

There is no need for either in a single circuit brake system as the components are matched for displacement. You would only need a proportioning valve if something has been altered, like changing over to a different caliper or altering the rear wheel cylinders. When going to dual circuit brakes and changing the master cylinder you’d definitely want a prop valve. I’ve never seen a check valve or residual pressure valve on any Brit car.

Warped front brake rotors will push the pistons back into their bores resulting in a “dead” pedal and requiring several pumps to get brakes again. I don’t know how air got into the system only in the back, but I’d check to make sure all of the brake pipe fittings and the bleeder are tight and that the connections for the flex hose are tight. if they are loose, air will be drawn into the system as the brake pedal is released.

• This reply was modified 1 year, 5 months ago by Sean Johnson.

Assume it is a standard stock brake system. The problem is air in system after sitting extended period of time. Sort’a like; worked when parked.

It is not likely the servo because it is a sealed unit. If servo leaks it is usually sucked into the engine.

If the car is stored in an environment where temperature swings are frequent, perhaps humidity high and Girling fluid is used, air and moisture will migrate past seals in the cylinders, including the master as corrosion develops.

Although air was extracted from rear cyl, it may have come from the master. It is not uncommon for master cyls to inject air as the seals wear and corrosion builds up in the bore from light corrosion deposits. Pumping the pedal tends to partially clean up the seals and make it functional again…..for a while.

Use silicone for vehicles used occasionally and parked for extended periods of time and these kinds of problems will not likely occur.

If any of the system is original, or used NOS parts, you cannot use dot 5 silicone fluid as it is incompatible with natural rubber seals. If you have a new system, it must be complete cleaned with alcohol before switching to Dot 5 as Dot 4 and Dot 5 have different swelling properties and cannot be mixed.. if you don’t you will soon see black fluid in your reservoir… the black is the seal particles that have left their “hosts”.

additionally, Dot 5 is compressible, while Dot 4 is not. you will always have a slightly “spongy” pedal when using dot 5 fluid.

I understand the attraction of dot 5 as it cannot absorb water. like dot 4. Periodic maintenance was once an accepted fact of owning a car… people have gotten lazy. Brake fluid should be flushed and changed at least every 2 years.