Reason I ask is that in some vehicles they are and lead to the same brake issues I am having. It's related to the Hydro-boost to the reservoir.

I'm a complete newbie, so I have a lot to learn about these cars.

 

This is the post referring to the hydro boost problem


Re: '93 Front Brakes Applying Themselves
Postby oldsafari [OP] » September 3rd 2012, 2:55pm

Update...did some troubleshooting yesterday. Jacked up all 4 wheels and checked how much drag was on brakes before starting. Slight drag on front (normal). Started engine and without touching brake pedal, checked drag again. Drag on front brakes definitely increased, no change on rear brakes. Revved up engine and turned steering wheel back and forth and drag on brakes increased even more, but not to the point of locking up. This vehicle is equipped with hydraulic brake assist from the power steering pump (Hydro-boost?). Reading the Chilton's manual, it mentions a restriction in the power steering return hose from the Hydro-boost to the reservoir. I checked the power steering fluid level, and the reservoir was almost empty and below both hoses. Pulled the return hose and fitting off of the Hydro-boost and saw that the fitting had a small orifice. I cleaned out the orifice and anything i could out of the Hydro-boost connection. Re-connected everything and topped off with fresh fluid. Started engine again and did not see any increase in drag on the front brakes. Drove around for an hour with no problems.

I am thinking the drag produced from the Hydro-boost eventually overheated the front brakes to a point where the fluid began to boil and continued increasing the pressure on the calipers until the brakes locked up. I think next weekend I will flush all of the brake fluid and maybe the power steering fluid as well.

 

Just found out from Kirby Palm that this brake system does not use vacuum, so no replies needed to this post.

Thanks again Kirby for your help!

 

75gremlin,

That's not strictly true. The xjs did move back to a vacuum-driven servo. However, your 93 car doesn't have the servo fitted.

Hope that helps.

Paul

 

OK, my mistake and should not reflect on Kirby.

 

Absolutely not. His knowledge is unparalleled on the earlier cars.