I need to understand what is going on with my '99 V70 brakes. Most of the time they work just fine, but a couple of times each trip when I apply the brakes the pedal is very high and very hard, with no apparent braking effect. I then must pump and/or press harder to get the brakes to resume normal operation. I'm guessing the problem must be one of 3 possible components - the check valve, the position sensor, or the booster itself. I don't want to replace all 3 and discover the problem is something else. I'd like to understand why it is happening before throwing time and parts at it. Car has ~100k miles on it and is all original. Suggestions please?


Thanks for helping!

 

Not a factory trained tech but that sounds like an intermittant loss of vacuum. Does it do that while driving, decellerating or after a stop? Ever hear any hissing? I'd suspect the check valve/lines or possibly the vacuum pump first. Not sure how the booster can fail intermittantly and not sure how the pedal position sensor would influence the power assist.

 

I suspect something like that check valve but don't understand exactly how it works or how to diagnose without replacing it. No vacuum pump on this care AFAIK, No hissing, no fluid loss, no driveability/running issues, no consistency in when it occurs, just sometimes when putting on the brakes it's like someone put a block of wood under the pedal. THANKS for the input!. Bob
If someone here has a known-good check valve I'd be interested in picking it up to try.

 

Here's what I'm looking at. Would like to try the check valve first, Anyone have a spare?

 

I think you're on the right track. The only thing connected to that pedal is the booster. The booster is not able to fight back against your foot based on the input from any other components. It just is not working. It either needs to be replaced or else it has some means that it looses all vacuum some of the time. Check valve failure could do that, sort of, but it's not entirely believable. usually, when you hit the brakes, there is plenty of vacuum at that moment if the engine is running. The checkvalve mainly gives you one good solid brake application after the engine dies. That is its main job.

 

this is exactly what happened with mine only once it started it was consistent not intermittent. did you solve it? my car is out of commission until I can

 

do you hear your vacuum pump coming on when you first start up? do you hear a hissing noise when you press the brakes (with the engine running)? Does the brake warning light ever come on? Do you have brakes at all? usually a failed booster means you will have to press the pedal harder but you can still stop. Otherwise if the pedal feels normal but needs to be pumped to build pressure to the wheels, I'd be looking at the master cylinder having an internal leak. Assume you checked fluid levels and inspected calipers for leaks...

 

I'll check both those things now, thank you much! just got a charger for the battery that happened to die during this fiasco. no leaks, fluid at a fine level, I have a thread recently opened just below this one that has all the details

 

After replacing the check valve with no improvement I finally resolved the problem by replacing both front flex hoses.