I have 2000 Disco, with 100G miles. I just bought it. Put 4 O2 sensors, replaced all fluids. When starting the truck, the brake booster sucks the pedal down.

It all started after a precipitous climb down a pass in the Rockies on a logging road. There was no trouble going down, but the next day, I started the truck and I had no brakes, no leaks, no dash warning lights. I decided to head home and in the 2 hours to get home (no braking required --thank goodness), the brakes came back. Then while parked over night, they left again. I bought an OEM master, bench bled on the vehicle (with a rag under the unit), bought a mightyvac, confirmed the vacuum hose was ok and I can pump, and pump, and the bubbles continue. Again, there is no fluid loss, and I now have a dash warning light on. I am at my wits-end, having an MG and a Jag, and having bled both of them with no problems. Could there be an ABS problem? ( I looked under the driver's seat and swapped out a rely with one in the engine and there was still, no pressure.) I do not know if the ABS is working). Thank you for your expected assistance.

 

"bought a mightyvac, confirmed the vacuum hose was ok and I can pump, and pump, and the bubbles continue." If you keep bleeding it and you keep getting air bubbles then it is sucking in air from some place, loose fitting or bleeder.

One other thing have you looked at the check valve on the brake booster, the reason I ask is you say your brakes came bach? but after sitting over night you lost them again, it could be a vacuum leak in the check valve or booster.

 

Thank you. I connected the mityvac to the hose off of the manifold (to the brake booster) and gave it 15 psi and it held it for over an hour (without starting the truck). I found a great post from years back so I will (1) start the truck, idle it and note the reading; (2) shut it down, disconnect the vacuum line to the booster. plug it on the manifold side and take another reading. If it is a higher reading then I will replace the booster. If not, then continue with someone pressing the brake. Another night in the garage.....How I miss my 58 Series One pick-up.....

 

Shout if you verify it is the booster, there are a number of good guaranteed used units on the market.

 

Thank you Disco Mike. The testing tonight showed (when running): (1) 12 psi from the manifold; (2) nothing from the booster; (3) so with the guage connected to the bootster, with the brake pedal depressed, 3 psi, with the pedal released, no change. So using the test posed by DarylJ (senior administrator): "if it returns slowly to the original level (before you hit the brakes) you have a collapsed or blocked vacuum line or a bad 1-way valve (aka non return valve). If it doesn't it is a bad booster". I did not hear a hiss or hum from the booster. Did I follow instructions properly? If I do need a booster, thank goodness Disco Mike came to the rescue as the OEM boosters are tres chere! I am off to your great country to get some R&R in Helena MT, so I look forward to hearing about sources for rebuilds, unless any of you disagree with the analysis set forth above. Cheers, Kevin