Low Brake Pressure (Soft brakes)

My 1988 Vanden Plas uses the hydraulic mineral oil system for the brake booster. Recently the brakes became soft when pushing on the pedal, and the “Low Brake Pressure” warning appears on the Vehicle Control Monitor (VCM). When I checked, it needed mineral oil. After I filled the reservoir with mineral oil I soon discovered a leak from the plug on the side of the reservoir. I repaired the leak by replacing the plug, and then filled the reservoir again with mineral oil.
The brakes are still soft, and the “Low Brake Pressure” warning still appears on the VCM. Although now when I pump the brake pedal twice I get good pressure on the second pump.
I am thinking that air might have gotten in the system from the leak in the reservoir, and that bleeding the brake system will get brake pressure back to normal. What do you think?

Can’t do any harm. Try it and report back.

The reservoir for the mineral fluid is self bleeding. The system you are about to bleed was unaffected by the work you did, the fault is on the hydraulic side I.E. brake booster or your accumulator

John W., Thanks for your reply. I get good pressure after pumping the pedal once. If the brake booster were bad would I get good pressure after pumping the pedal?

the most common problem in your 88 is the accumulator, most owners have converted their cars to vacume brakes. you may need to read up on the Ebooks area of the site regarding brake issues and accumulators.

wish I could find a straight forward answer to the conversion, part numbers and such, mine has been out for far too long! If my engine dies so do my brakes, not good.

Yes I agree. I’ve decided to have the system converted. There is just too great a chance of problems mounting with the hydraulic system.

If you find all the parts let me know exactly what you used. I really want to upgrade but just to lazy to locate the various parts.

This should help.

Jaguar Brake Boost Conversion.pdf (1.28 MB)

That helps alot, thanks.