Ok, so in the middle of changing the head gaskets on the 2000 Disco, the 2002 disco I bought to replace it develops a leak in the brake booster (hissing sound when brakes pushed). So I swap the booster for the 2000 in the 2002, no issues.

Finish the head gasket job, buy a replacement booster, and put the 2000 back together. Engine runs great, brakes not so much.

Pedal is hard and firm but acts like booster is inop as in no power assist. Swap boosters back between trucks, no change. 2002 still works great, 2000 hard as hell.

I'm at a loss what could have happened to the brakes on the 2000?

Searched the forums, someone mentioned a seized caliper so I pulled the front calipers and checked, both work free and easy.

3 amigos is on but had been on for weeks before head gasket blew.

Saw a mention on the pushrods adjustments, but I am thinking that must not be it because I have swapped boosters back and forth with no change - unless that affects how the internal valves open on the booster (does act like the booster doesn't work - even though it does on the other truck)?

Any advice from the experts?

 

Is there any difference in brake feel when the car is running vs when the car is off? Sure sounds like a booster problem. check and make sure vacuum is getting through the 1 way valve, may be as simple as that.

 

x1. The brake pedal would normally pump up solid without the engine running. To have power assisted brakes from the servo operating you need the engine running and a vacuum being pulled to the servo to actuate it. You may know this already but no harm in mentioning it again. As RoverMasterTech states, there is usually a small NRV inline to maintain the vacuum when you come off the throttle and this valve may have jammed closed.

 

Yes, checked vacuum at the booster with the engine running - good vacuum at the booster fitting. No difference in feel running or off - brakes don't pump up. In fact, normally with power brakes when you push on them with the engine off, then start the engine the pedal goes down a bit as the booster leverage lets you push the pedal a bit harder - that does not happen.

I'm stumped - pushrod adjustment inside the booster? It feels like the booster internal valve is never opening - but like I said I have swapped boosters back and forth with no change. Anyway the master cylinder could do this?

 

I may be stating the obvious, but did you get the brakes bled completely? You may try that again to make sure all of the air is out of the system.

 

if all you changed was the booster there should be no reason to bleed

 

I would bleed the brakes to start with. This should show you if any fluid is being pushed when you press the pedal and it will also give you a chance to have a look at how dirty the fluid in the line is.