Hydor boost brakes on a 1990 Mallard Overland

hi,
this sounda to me like a problem in the master cylinder.sounds like one piston is bypassing.try to bleed system see if you get pressure at all 4 wheels.hydro boost is only an assist unit,problem here should not cause a low pedal.may cause high pedal effort or leak but not low pedal. good luck

duckie does the pedal almost go to the floor with engine off?

hydroboost may be your problem...but you could have frozen caliper slides, soft expanding brake hoses, soft cheapy brake pads

did brakes work properly then all of a sudden begin with this problem?

If it is a class C and has sat for a while...they are known for sticking caliper slides due to water off on the frt corners of the roof

Your rubber brake lines are probably going bad. Get then replaced and get the brake fluid replaced also.

You will need to get the Mallard into a shop to inspect the brake system. Bleeding the air out of the hydraulic brake fluid may be the only thing needed but you should have it inspected. If the vehicle can't be driven, you could have a mobile tech inspect on your site or have the vehicle towed. I would not drive it.