The car (2004 V70) was in need of brake pads so I did all 4 corners. I installed newly turned rotors in front. Rear were looking OK so I used the rotors as is.


To install pads, I pushed the pistons all the way back in, put the new pads in & reinstalled the calipers.


Next (with the engine off), I did the typical bleeding routine. Assistant pumps the pedal & holds, I open a bleeder, pedal drops, close the bleeder. Repeat about 6-8 times each being watchful of the reservoir level. I refilled the master cylinder each time so the level never dropped below normal.


All seems well. The assistant (who has done this many times) thought the pedal behaved as usual.


Thinking I was all done, I started the car to see how the pedal was with the power assist running. To my surprise, the pedal goes to the floor in about 2 seconds.


I gave a try at another round of bleeding, this time with the engine idling. Still no good.


Next I wanted to see if it behaves as before with the engine off. It does. The pedal is hard & drops only when a bleeder is opened.


Now, for full disclosure.... for a while, the pedal on this car seemed a bit low, but has been consistent for about 4 years.


Thoughts,
Mike

 

It can be misleading to test the firmness of a brake pedal with the engine off - you're applying so much less pressure that you won't feel a lot of problems that become obvious once the full vacuum assist comes into play.

It's possible you have a bad master cylinder, or perhaps just ended up with air in one of the calipers (hey, it can happen in a heartbeat, even if the level in the reservoir never really drops to the bottom during the bleeding process - air can get sucked in through the last 1/4" of fluid on some reservoirs).

If the brake pedal feels pretty firm when you first stomp it with the engine running, but then obviously drops without any additional input, I'm betting it's a bad seal in your master cylinder (letting fluid move from the output side to the input side). If it was a leak anywhere else, it would become obvious pretty quick (by the big puddle of brake fluid and the near-total lack of brakes). It could be that it got worse just because you put more pressure on it than you have in a while, or maybe the new fluid didn't agree with your old seals (make sure you were using the right fluid, of course).

 

Usually, when installing new pads, it's better to push back the piston only after opening the bleeder. This way there is no back pressure on the master cylinder that could damage some valve.

As for bleeding a caliper, there is no need to close the bleeder each time the pedal is released. As well, I would be careful when depressing the pedal to bleed a caliper. Never push the pedal all the way down, better depress it only one inch or so. It is also a good idea, after opening the bleeder to first push the piston back. This way, all the old oil inside the piston is pushed out in the first place, making for a better bleeding of the old oil.

 

Problem is solved. I did require that the master cylinder be replaced. It makes sense that it had to be.


Because, one certainty is .... If the pedal drops slowly under pressure & there is no loss of fluid or visible leak; your problem must be internal to the master cylinder.


As to how it got damaged, hard to pinpoint, but there a two things I did that I will avoid in the future;


1.) I pushed the caliper piston in without opening the bleeder. Next time the bleeder will be open so old fluid come out instead of finding it way back into the system.


2.) I could have the assistant avoid pushing the pedal all the way down. On the other hand.... If the master cylinder is going to fail because of being stroked too far... it might as well happen in the garage rather than on the street.

 

For what its worth, I'll add my experience. The brakes are SOOOO much easier than the last system on an older Honda featuring captive rotors (holly heck was that a headache!). I bought all the parts in stock from a local Carquest ($60 per rotor and 42 for ceramic pads for the axle). DOT 3/4 fluid.

So, luckily when I compressed the pistons I didn't destroy the MC (I just did the front brakes this weekend). However, next time, and when I replace the rear soon as they are noisy as all get out, I will break the bleeder open prior to spring compression. I ended up using a acetylene torch to heat up one bleeder valve as it was seized. The other three broke lease pretty easily and I added anti-size to the threads for next round.

One thing I did notice, was that the order of brake bleeding I thought was correct (farthest from MC working toward the MC) was incorrect. Apparently according to VIDA, the correct order is LF, RF, LR, and RR. I had a huge bubble the first time around and the pedal went to the floor. Re-bleed in the Volvo specified manner and pedal is firm.