Losing Brake Pressure at Low RPM

[mrt740]

Had this happen 4 times now. When rolling into a parking lot at low speed and RPMs the brakes will feel fine then suddenly the pedal goes to the floor and I get barely any braking force. Issue only rarely happens but could be serious if at the wrong time. Then replaced my rear brake pads last weekend due to failing inspection for them. Rebled every caliper and brakes seemed fine for a few drives until pulling into my neigborhood last night brakes went out and I nearly missed the turn. Then again pulling into a parking spot, brakes went out and I rolled 2 feet until the tires hit the curb. I am going to rebleed the brakes with a pressure bleeder today but as the issue happened before ever messing with them I have a feeling it isnt just air in the line. Anyone experianced this or have any ideas what could be going on.

 

[Pietercc]

I have had my master cylinder start leaking while bleeding

 

[2turbotoys]

The model and year would help. A lot of 240s have a fairly complex bleeding order. If you have ABS, its a pain to bleed. I assume there was no warning light light up. Wooshing noises when braking is a failed brake booster.
Make sure there are no leaks front and back of calipers, a friend of mine recently took the wrong bolts out when changing pads and the caliper started leaking. I had a caliper piston start leaking out of no where once, and didn't know what the issue was till it got bad enough to get onto the wheel.

 

[MasterBlaster]

Had this happen 4 times now.
Then replaced my rear brake pads last weekend
Rebled every caliper and brakes seemed fine for a few drives

Did it start happenning after the pad replacement and bleeding, or before?

If after, the bleeding could have caused the problem.
A master that only get pushed half-way for most of its life will tend to collect residue and dirt in the "un-swept" area of the bore. Shoving the pedal to the floor while manually bleeding can force the rubber cups inside the master to grind into all that dirt and crud, possibly damaging them. Pedal dropping to the floor without external fluid loss is a classic symptom of a failed master.

 

[mrt740]

It has happened twice before replacement and bleeding and twice after. Only reason I don't expect it to be the master cylinder is the issue is so intermittent and a leaking master id expect to see the symptoms somewhat regularly.

 

[Radtap]

How's the brake booster check valve

 

[towerymt]

How's the brake booster check valve

Worth checking. And the hose too. But pedal to the floor sounds opposite of what I'd expect if it was a loss of vac assist. Firm, high pedal, and feels like it's not braking enough given the pressure applied to the pedal, simply because the vac assist is lost.

If you hold gentle-to-firm pressure, but not stand-your-weight-on-the-brake-pedal pressure, does the pedal sink slowly to the floor? I've not felt a failing master cylinder since I first started driving, I think, but I was told that the pedal will sink, like if you're sitting at a red light, stopped, and holding the brake pedal gently, it may sink if the MC is bypassing fluid internally.

You might also check for fluid leakage out the back of the master cylinder where it bolts up to the brake booster, but I don't know for sure if there would be any fluid present. You might still have a MC issue w/o finding fluid there, is what I mean.

After my last brake saga with my '93 245 (non-ABS), I would also suspect the booster. I swapped on new front calipers (3-screw, non-ABS, same as what came off the car) and after bleeding, I had a low and soft brake pedal all the time when driving (when engine running). Engine off, brake pedal felt close to normal, but start the engine, and the pedal was very low, just off the floor. Brakes worked, it stopped OK, but the pedal travel was LONG before it started braking. It was not particularly squishy like air in the lines.

The car went to a Volvo shop for a month, they bled gallons of fluid through the system, couldn't fix the issue, replaced the master cylinder they said was leaking out the back, didn't fix the issue, and I got the car back in the same condition I had dropped it off. Low, somewhat squishy brake pedal.

I swapped the booster and found the reaction disc was bouncing around inside the booster. I had swapped THAT booster on the car earlier trying to shotgun the issue and not having any success. I could have had two boosters with similar issues...not sure...but the 3rd brake booster finally solved my issue. Normal height brake pedal again. And no more near-stalling when braking to a stop (automatic trans). The old booster liked to dip the idle to almost stalling, sometimes, not every time.