I know there are about a million threads on this topic and I've read through almost five hundred thousand of them. However most of them seem to black hole without a resolution. So hopefully that won't happen here.

Back story:
Friend brings me a 95 850 non turbo complaining of bad brakes and wanted to do the pads and rotors. So we install new ones all the way around. Car needed a lot of other work to get it in satisfactory condition and we decided to do a little at a time. At this time we could tell that the brake system was going to be in need of more work than just pads and rotors due to the hulk type strength you needed in order to stop the car. It had á very hard pedal that sunk pretty low to the floor. However the car was purchased as a type of grocery getter so we weren't too concerned about it at the time.

Now jump ahead a couple of years the car has proved it's worth and money is being spent on it now. First order of business is the brake system. It still had the hard and low pedal but after a couple years use the pads are barely at half life and rotors look good as well. And everything shows even wear.
So we start with a booster change which makes worlds of a difference.(no longer have to clutch the steering wheel tighter in order to provide the needed leverage too force the pedal down to stop the car) now it feels like it has power brakes again and you can stop the car with foot and ankle strength.
Few things I noticed after the booster change was that it had pulled the pedal a little further toward the firewall, as i had to adjust the brake light switch In order to keep the lights off when my foot was off the pedal. Also brake fluid was dark so I flushed the system. These two steps were done before the first test drive after booster change.

so now we have what feels to be power assisted brakes but still a low brake pedal.
I continued by bleeding the system a second time which produced minimal improvement, then changing the booster vacuum line since I found a leak in it, also a small improvement in braking performance but still a low pedal in my opinion.
And that's where I stand
I did bleed the system a third tiime just to make certain there is no air in the lines, of which only fluid came out at all four corners.
I do notice while driving if I'm going at a steady speed(say 35) and normally press on the brakes the pedal goes low and car start to slow down, however if while coasting on the brakes while slowing and quickly release and again depress the pedal it stays high and firm like a good brake system is suppose to feel. This only lasts that moment. Once I accelerate and brake the next time its back to a low pedal. It seems like a good two stroke of the brake pedal gives me the type of brake feel that I'm looking for.(just want to make this happen in one stroke tho).
Any suggestions on what to look into next?

My thoughts lead me to possible air in the abs system, maybe pedal sensor in the booster(is still original), maybe a hanging caliper(although everything looks even)???

I'm ruling out master cylinder cause the pedal doesn't exactly sink. It goes low and stays there. No fluid loss externally.
Also want to mention abs light popped on recently rf and RR codes. I think is wiring tho. Have to look further into it.

 

If you haven't done so recently take all four calipers off, clean and lube the slides and or pins so each caliper can follow it's rotor. With the caliper off inspect the pads and rotor surface with something perfectly straight and check in two directions. Check for ridges and valleys in the rotor surface and the pads surface. Rotors can be machined flat but if the pads are uneven they need to go no matter how thick they are.

This is based on you knowing you have all the air out of the system. Uneven pads or rotors will cause a soft or low pedal as you aren't getting full contact and you are actually bending the pad to get it up against the rotor and as the rotor spins it's pushing back with the high spots.

You might still have a marginal master cylinder that is only sealing and producing pressure in one chamber and not both. It can do that and still not be leaking. It's just that one of the seals is rolling over or just bypassing fluid internally.

 

That's the plan for this evening. All calipers off an maybe another bleed.
What ur say about the MC makes since. However if only one is sealing then why would you say I get a high pedal after a quick two stroke of the pedal? Does one side have the ability to pressurize both circuits?

 

Nope, can't pressurize the second circuit if a seal is bad. BUT, the first part of the stroke of the pedal is to push fluid into the caliper to push the pad out to the rotors face and then to apply braking force. If you let up and go back down quickly some of the fluid used to push the pad to the rotor hasn't had a chance to bleed back into the master and the masters reservoir so when you press the second time it takes less fluid and your pedal feels higher because you're using some of the fluid from the first application.

 

What about brake fluid loss, period? Are you losing any brake fluid?

Also:

1. Was there brake fluid in the old booster? If yes, it came from the BMC. When i was a tech, if you went through all the trouble to replace the booster, would generally replace/rebuild the BMC too as a matter of course..
2. Are you bleeding with a pwr bleeder? If not, you're wasting your time IMO.
3. Have you ckd for proper brake pedal adjustment?

 

This seems to be a likely cause. I was thinking earlier that it seemed similar to the feeling when u finish a brake job and you need to pump the brakes in order to expand the caliper piston so the pads sit close to the rotor. Just not quite that extreme tho.
Anyway of testing to verify this? I'd hate to put a 395 MC in only to feel the same thing after.

 

Zero fluid loss...specifically at the MC. Thoroughly checked that when I removed it from the booster. Bone dry in between the two. Also brake fluid level has not budged since the last bleed. Nor was it low before the first bleed just dirty.

I'm two person methoding it...lol. no power bleeder yet. Although this car might prompt a purchase

Brake pedal adjustment??
From what I can tell the booster dictates the pedal height correct?

 

Well all calipers came off and all pads and rotors are straight. All pistons moving freely, and all hardware lubed.
Abs error code repaired. Wiring fixed.
Still the same feel of the pedal, so looks like I'll be changing the MC today. Any other possible causes?? Is there any other way I can check the MC to be certain of it??? I mean at this point there isn't much more left to the brake system anyway but just want to be thorough.

One thing that pops into mind is the secondary vacuum pump for the brakes that some later model Volvos have. Did these 850's have those? From the way the booster vacuum hose was routed it doesn't look like they were designed to have them.

 

NNew MC installed. Feels like a pretty normal brake pedal again.not quite the type of pedal feel that I'm looking for tho. still notice a higher pedal after a double pump. I'm guessing probably the brake hoses could use a changing their pretty old. Maybe some higher quality pads also. But that's it for now. Car needs some other things done so if I see it again I may go further into the braking system at that point.

 

Slightly higher pedal after 2x pump is normal. Think about it, you're taking up the space/slack between the pads and the rotors on first pump..

I drive that way normally (2x pump) as it not only gives you better pedal, but also get's the driver behind you's attention..

Glad the MC fixed it..

 

One thing is for sure: new pads can't have any effect on pedal feel. They affect where you stop, but only by a little.

I know you know that, but some are learning.

 

My issue may be that I'm comparing it to two other cars. A 99 s70 and a 08 v50.
This is a noticeable difference of braking strength after the second pump. As I mentioned before the second pump gives me the brake feel that I desire for the first stroke.
In other cars no matter how much I pump I still have similar pedal feel as the first stroke.

If I go back in to it and find something else then I'll post an update.