2004 xc70 no brake boost/assist

Post by **steve850** » 08 Sep 2023, 18:40

Hi all - can't believe I haven't been here in 7 years! Doing a full brake job on my old 2004 XC70. Replace pads and rotors up front, everything in the back. All good but the pedal was creeping to to floor, changed the master cylinder, bled over a litre of new fluid through the system (yes, including bleeding the master first) and had a hard pedal, with an inch of travel where there are no brakes. Start to suspect the booster. There is vacuum, but the pump runs all the time. If I pull the valve off the booster and cover it with my thumb, the pump stops. Change booster - used (what fun!) Just finished reassembly and have the same condition. Hard pedal, no brakes for the first inch, pump running all the time. No hissing inside the car, no whistles or anything like that. What am I missing? Thanks to all the great people here!

Post by **firstv70volvo** » 09 Sep 2023, 13:11

steve850 wrote: ↑08 Sep 2023, 18:40 Hi all - can't believe I haven't been here in 7 years! Doing a full brake job on my old 2004 XC70. Replace pads and rotors up front, everything in the back. All good but the pedal was creeping to to floor, changed the master cylinder, bled over a litre of new fluid through the system (yes, including bleeding the master first) and had a hard pedal, with an inch of travel where there are no brakes. Start to suspect the booster. There is vacuum, but the pump runs all the time. If I pull the valve off the booster and cover it with my thumb, the pump stops. Change booster - used (what fun!) Just finished reassembly and have the same condition. Hard pedal, no brakes for the first inch, pump running all the time. No hissing inside the car, no whistles or anything like that. What am I missing? Thanks to all the great people here!

Bad used booster or possibly crack in vacuum hose that closes up when unplugging it bending to plug with thumb. The inch of travel may be the booster rod to master cylinder adjustment. Some new master cylinders come with a gauge or you can purchase a brake booster push rod measurement tool for ~$15. This adjustment is important and may be needed when installing a new master cylinder, especially if it's not an OEM part. One test you can do to know if it's the booster is to see how long the booster holds a vacuum after turning off the car. Turn the key off and press on the brake pedal after different amounts of time. A good brake booster will hold the vacuum for a very long time so if the the brake pedal is hard on the first push of the pedal after turning off the car you've lost vacuum or didn't have it to begin with. The vacuum will be used up as you press the pedal more than once with the car off.

Post by **jonesg** » 09 Sep 2023, 16:09

can it be smoke tested, just curious.

Post by **steve850** » 09 Sep 2023, 16:33

I thought of that, I tried blowing into the vacuum line and there is resistance after a second or two, but I know I’m not moving as much air… time to get out the home made smoke machine I made years ago (requires a cigarette in a glass jar, lol

Post by **steve850** » 09 Sep 2023, 16:38

Firstv70 - I’ve been through all the vacuum lines, everything looks good. I’d call it a hydraulic problem if it weren’t for the pump running all the time. There is no crack in the foot of hose before the booster, and you can feel strong vacuum when you pull it out of the booster. There is also good vacuum off the intake.

Post by **firstv70volvo** » 10 Sep 2023, 09:15

steve850 wrote: ↑09 Sep 2023, 16:38 Firstv70 - I’ve been through all the vacuum lines, everything looks good. I’d call it a hydraulic problem if it weren’t for the pump running all the time. There is no crack in the foot of hose before the booster, and you can feel strong vacuum when you pull it out of the booster. There is also good vacuum off the intake.

If you can remove the vacuum hose from the booster, plug the end with your thumb and the vacuum pump then stops, there has to be a leak with the brake booster or at the hose to booster connection. For the one inch of travel, did this problem show up after the pad and rotor change and before the booster replacement? If so check the installation of the brake caliper spring clip on both front calipers.