Sinking brake pedal mystery
 

My new to me 2002 has been hanging around various shops for days with a sinking brake pedal. The brakes work fine but the pedal goes to the floor, or close to the floor, during application.

The system has been bled multiple times, the last time was with a power bleeder and a number of new master cylinders have been installed. The calipers appear to be functioning properly and the computer shows no errors in the system.

In looking through this and other forums, the smoking gun always seems to be air in the system, a bad MC or mis-installed clips on the front caliper.

Are there any other things that might cause this behavior? Could the ABS be involved?

Sounds like a brake booster has failed.

that was my thought initally becase the actual brakes are working fine, but the mechanic felt that a failed booster would lead to a hard pedal, not a sinking pedal?

There are only two things ahead of the brake pedal, the booster and the master cylinder. If you are satisfied that the master cylinder is holding pressure, then it has to be the booster.

One way to diagnose this issue is to hold the brake pedal and let it hit the floor or maximum travel. Then, while holding the pedal down, have someone open a bleeder screw. If you get very little fluid, then it is somewhat safe to say that the master cylinder is bypassing. If you get a large quantity of fluid, then the master cylinder is holding pressure.

I've not had a Volvo brake booster apart.

thanks for that trick. Can I bypass the booster for testing by removing vac hose? I'd hate to replace it just for fun.

No. There is still a mechanical connection inside the booster.

so if that's the case, would a failed booster result in a sinking pedal?

any wheezing or hissing sounds when you pump the brakes?

It's been a while but the problem is finally solved. A third mechanic replaced the ABS motor, power bled the brakes twice, isolated individual calipers, etc. etc.

The fix?

A 5 dollar check valve on the booster

Thankfully, the seller dealt with all this nonsense. Now I own this high maintenance trophy wife of a car, hopefully it won't break my heart.

Exact issue
 

I know this thread is older than dirt but I have this same issue. Anyone have any idea what check valve this would be ?

I would also like to know what the $5 fix is!

After replacing the master cylinder and booster in the last two weeks, my ‘04 XC90 is still experiencing a strange incremental depression of the brake pedal when fully stopped, like at a light or in traffic.

After the master was replaced, a strange hissing when pressing the brake pedal while in park started happening.

Right after the booster was replaced, I heard a little puff of air while gently braking and then 15 min later a “brake service assist required” message came across dash. Even though the brakes feel like they’re working and I don’t feel other changes in the functioning of the car.

(New to this forum in general and just realized this is a s60 & v60 thread, but I haven’t yet read about a similar phenomenon anywhere else)

https://www.fcpeuro.com/products/vol...0-xc90-9473812

that would be this part right here. connects the hose from the brake vacuum pump to the booster itself. Easy to spot, sits at the bottom of the booster.

This is for the 05+ years.

EDIT: there is also a check valve between the intake and the vacuum pump valve (switches where the vacuum is pulled from whether the intake or the pump) Inspect these parts for proper function.

Same brake problem on a 2010 xc60
 

I'm having the same issue with a 2010 XC60. I'll be looking for that check valve thanks