intermittent loss of braking power - PeachParts Mercedes-Benz Forum

funola · #1 10-02-2011, 04:23 PM

I was doing highway speed and used the brakes to slow down and it felt like it lost 90% braking power. I pulled into a fuel station and had a quick look. Brake fluid level was fine. There was vacuum, (pulled off a vac line off the main stiff plastic vac line and felt suction with finger). I reseated all the vacuum connections in the engine bay and got back on the road and still had weak brakes but after a few miles of driving the braking became normal. When I got home measured with vacuum gauge and got 19" of HG at idle. Have not driven the car since. What do these symptons indicate is the culprit? What should I check next?

leathermang · #2 10-02-2011, 04:40 PM

Contaminated brake fluid. Like Moisture......
Flush and bleed entire brake system with proper NEW out of can fluid...

Biodiesel300TD · #3 10-02-2011, 04:59 PM

Could also be a weak vac pump. I had problems with my 240 having no power brakes if I used the brakes three times in a row within a short time period, less than a minute. The third time I'd have no power assist. The vac pump was pulling over 20", but when I applied the brakes it couldn't keep up and vac would drop to 0 after a two uses. If I waited a short time it would build back up and work properly. So I'd test your vac pump, but pressing the brakes with the car running a couple times and watch the vac.

tangofox007 · #4 10-02-2011, 05:17 PM

Intermittent loss of power assist is a fairly common symptom of a failed vacuum pump check valve (in the nipple where the hard line attaches to the pump.) The check valve parts rattle around in the pump inlet, causing an intermittent blockage.

funola · #5 10-02-2011, 08:54 PM

Brake fluid is a year old and looks clear. I will tee in a vacuum gauge and watch it as I use the brakes and see where it's at. How does one check the brake booster if it's good or not?

vstech · #6 10-02-2011, 09:15 PM

booster can be bad in a few ways, but it sounds good to me, it does sound more like a vacuum pump issue. pull the checkvalve if the testing shows low vacuum after two pedal presses.

funola · #7 10-17-2011, 08:59 PM

Vacuum pump is fine. It's putting out 20" HG. The check valve tested fine also with a Mityvac- holds 25" HG in one direction. I bled the brakes and replaced the front pads. One was worn down to the metal. I think that pad must be hanging up. The brakes feel better now but is still not 100% IMO.

What is that piece in the center of the pic and what does it do? It's rusted out and fell off one of the pistons when I pulled the pads.

In the bottom of the pic is my pressure bleeding cap I made for the job. I went to a tire shop and picked up a valve stem off the ground, drilled a hole in the cap and shoved the valve stem in like in a tire rim. I used a hand bicycle pump for pressure, works great, just have to watch the level and refill before it runs low.

charmalu · #8 10-17-2011, 10:07 PM

Got your money`s worth out of those pads. I guess you are not using the brake sensor wire? the dash light would have come on if you were.

Charlie

Yak · #9 10-17-2011, 10:23 PM

Rusty part in the center of post 7 is the heat shield. Available as part of the caliper rebuild kit as seen here: PeachParts FastLane Mercedes Parts Store

Maybe available separately. I'd check the rubber dust cover condition on the calipers if the heat shield was in that poor shape.

With pad wear that asymmetrical, I'd be looking for a stuck caliper piston, too; or maybe an old caliper seal that doesn't assist with retracting. Did the new pads go in easily? No issues compressing the pistons?

barry123400 · #10 10-17-2011, 10:34 PM

I was just wondering if a dragging brake caliper at hghway speeds might heat the fluid enough to have boiled. Therfore introducing air until the fluid cooled down. Check your wheel temperatures next time out to see if one is really hot after a good drive if your symptom continues.

Boiling temperature of brake fluid depends on the dot number. 400 to 550 degrees F covers the normal spread of brake fluids boiling points when new I think.

Anytime I see one pad on a two piston opposed caliper worn down far further than the otherside I suspect a dragging piston. Sometimes I suspect as a pad gets really worn down the piston goes a little off and tends to stick more.

I know by design that this should not happpen but I still wonder. I usually throw my ir gun in the dash to check temperatures. Much more sensitive than the finger on a wheel. I directly read the disk temperature after coasting to a stop.

funola · #11 10-17-2011, 11:14 PM

The pads are less than 30k miles old. Brake sensors in-op otherwise would have gotten a warning light. My M.O. was to replace pads and flush the fluid every year. One of the pistons in a caliper must not be retracting fully. I'll be looking for another set of calipers. Maybe the set from my 240D parts car will be in better shape.

tram · #12 10-18-2011, 02:31 AM

Don't condemn the caliper right away. Drive the car and see if that wheel is getting hotter than the others (don't burn yourself feeling- be careful!)

Then, put the front end up and see if the wheel with the suspect caliper is more difficult to turn.

If it is, open the bleeder. If the caliper relaxes and the wheel loses its drag, replace the flexible hose. If not, replace the caliper and replace the hose anyways.

Maki · #13 10-18-2011, 11:25 AM

Funola, I have a brake bleeder cap just like yours.

I agree you're due for some calipers. If you try the ones from your parts car, examine the outer boots on the pistons carefully for signs that they aren't fully seated or even cracked. Moisture can get behind them very easily, and before long the caliper bore rusts up. It's pretty inevitable in the northern climate

I'd just get a pair of rebuilts -- and examine the boots on THEM closely.

funola · #14 10-18-2011, 11:34 AM

Lot's off good advice, thanks! Has anyone used POR-15 on the sliding surfaces of the brake pads? It's inevitable that they rust and bind to the caliper, especially if the car is not driven regularly. Maybe the POR-15 will prolong the time it takes for it to start rusting. May have to grind down the pads metal a bit since the POR-15 will add thickness.

Maki · #15 10-18-2011, 11:43 AM

I usually put a little brake caliper grease on the those surfaces. I think it "seasons" the caliper like oil seasons an iron skillet.