Brake pedal holds Pressure when engine is off...loses pressure when turned on

[Father Of Giants]

I bled the brakes in my my 300SDL. And I noticed that every time I cranked the engine on, I would lose pressure and as a re-bled the lines, air would be introduced back into the fuid somehow.

I did this three times.

Bled the breaks, till there was no more air bubbles.
Pumped brakes till the pedal was firm...engine is off.
Gave it a test drive around the block only for the pedal to lose SIGNIFICANT pressure.
Rinse and repead times 3.


So I'm trying to figure out why the pedals hold pressure when the engine is off, but as soon as the diesel starts rattling up...boom no pressure.

[Father Of Giants]

Just did some research and figured out that the master cylinder is 99% the cause of brake pedal failure.
This is the case because when I hold my foot on the pedal, it will slowly depress instead of staying put.

[Mike D]

A quick but not 100% definitive test of master cylinder failure allowing air back into the system is to bleed the left front caliper on a LHD vehicle. This is the shortest line and if air is entering the system, this is where it will show first.

The pedal slowly dropping under pressure is an indication of master cylinder failure as you have surmised.

[optimusprime]

Press the pedal down on the floor with out the engine on . Now start the engine still holding your foot on the pedal , it should lift you foot up off the floor to normal hight if booster is good. Also you have 2 sections inside the brake fluid header tank do make sure both are filled up at the same time with new fluid before you bleed the system .And keep both sections filled up throughout the sequence .

[Diseasel300]

Originally Posted by optimusprime

Press the pedal down on the floor with out the engine on . Now start the engine still holding your foot on the pedal , it should lift you foot up off the floor to normal hight if booster is good.

WRONG!!!!!!!

The vacuum assist is to help PUSH the pedal down, not pull it back up.

Without the vacuum assist, it is hard to get sufficient brake pressure to diagnose a master cylinder unless it is extremely worn. When the vacuum builds to operate the booster, it's MUCH easier to push the pedal down. Since you have more force available to push the pedal, smaller leaks and faults will show up.

A sinking pedal is a sign of a leak or a failing master cylinder. If you find no leaks anywhere (check caliper seals, hoses, ABS pump, etc) suspect the master cylinder.

[Junkman]

We don't really have enough information. You should never have to bleed a working system. Bleed to change fluid ok. Bleed after installing a part ok.


Why are you messing with brakes? What have you done besides bleed?

[Diesel911]

Originally Posted by Diseasel300

WRONG!!!!!!!

The vacuum assist is to help PUSH the pedal down, not pull it back up.

Without the vacuum assist, it is hard to get sufficient brake pressure to diagnose a master cylinder unless it is extremely worn. When the vacuum builds to operate the booster, it's MUCH easier to push the pedal down. Since you have more force available to push the pedal, smaller leaks and faults will show up.

A sinking pedal is a sign of a leak or a failing master cylinder. If you find no leaks anywhere (check caliper seals, hoses, ABS pump, etc) suspect the master cylinder.

Try it, it is a common test.
Although what I have read is you press the pedal down a 1/4-1/2 inch with the minimum pressure on the pedan and when you start the Engine and vaccuum gets to the Power Brake Booster the Pedal pushes back up.

The reason it does that is because when you forst start and Vacuum gets to the Booster the internal valves try to center the innards of the Booster.

[Father Of Giants]

I changed out the front calipers, rotors, pads, and brake hoses.
The master cylinder is shot, there are no leaks nor air the system. I'll buy an ATE master cylinder Friday and get this over with.
I stayed up and worked on the car from 2:00pm to 3 or 4 am in the morning.

Good news is no more brake squeak, car doesn't pull to the right when braking, and overall it's easier to keep in a straight line.
I fix a lot of things and botch the grand finale.

[Father Of Giants]

One last question, I have a power bleeder, what PSI is safe for a new master cylinder?
The Power bleeder can also be used to check master cylinder to see if it holds pressure correct?

[Diseasel300]

Originally Posted by Diesel911

Try it, it is a common test.
Although what I have read is you press the pedal down a 1/4-1/2 inch with the minimum pressure on the pedan and when you start the Engine and vaccuum gets to the Power Brake Booster the Pedal pushes back up.

The reason it does that is because when you forst start and Vacuum gets to the Booster the internal valves try to center the innards of the Booster.

I can see that with 1/4-1/2" pedal travel if you have a dual-diaphragm booster, but only in that case.

With a single-diaphragm or any more than the absolute bare minimum of pedal travel, pulling the pedal back up is detrimental, not beneficial to the operation of the booster's entire purpose.

Think about what was suggested above - if the booster has enough power to overcome you flooring the pedal without boost, what chance do you ever have of applying the brakes? It would be working against you, not for you.

[Junkman]

Originally Posted by Father Of Giants

One last question, I have a power bleeder, what PSI is safe for a new master cylinder?
The Power bleeder can also be used to check master cylinder to see if it holds pressure correct?

If you're getting a new ATE master 004-430-75-01-M4 I would bench bleed it by hand, install, connect my bleeder and pull the trigger. There is very little reason to mess around testing the new ATE master. I did have one that seemed to let thee pedal go to the ground. It was/is perfect and started behaving normally once the engine started and wheels were on the ground. Don't make work.

I have an extra cap to the brake master that I use with the bleeder. It will even set the correct fluid level. The inlet sticks down to the correct level. All that is necessary is to bleed, then release pressure , then pull the trigger and siphon the extra fluid back out.

[97 SL320]

Originally Posted by Father Of Giants

I changed out the front calipers, rotors, pads, and brake hoses.

Do the brakes feel OK when you drive the car normally?

With the engine off, does the pedal fall _all the way_ to the floor?

Does the brake system warning light come on? ( This is the one that monitors front / rear brake pressure and triggers is one system does not make pressure, it may be integrated into the low fluid / parking brake on light, check your manual. ) If not light, the MC is likely OK.

If driving is OK and from what parts you changed, the pads need to wear in so they sit flat against the rotor. Due to a build up of tolerances, you are flexing the pads , caliper mounts , rotor. Without brake assist, you are unable to flex the system enough to generate a low pedal.

The test for internal master cylinder leakage is to lightly and slowly press the brake pedal and see if it sinks all the way to the floor. It also helps to modulate the pedal , down 1/4" up 1/8" down 1/4" when testing for a internally leaking MC. Bonus feature, drive on a slightly rough road, rest the foot on the pedal, with a bad MC you will feel the pedal fall away as you follow it down.

If the pedal is pressed rapidly, the MC cups flare out due to pressure , seal against the bore and pedal hold height.

[Father Of Giants]

Originally Posted by 97 SL320

[/COLOR]Do the brakes feel OK when you drive the car normally?

With the engine off, does the pedal fall _all the way_ to the floor?

Does the brake system warning light come on? ( This is the one that monitors front / rear brake pressure and triggers is one system does not make pressure, it may be integrated into the low fluid / parking brake on light, check your manual. ) If not light, the MC is likely OK.

If driving is OK and from what parts you changed, the pads need to wear in so they sit flat against the rotor. Due to a build up of tolerances, you are flexing the pads , caliper mounts , rotor. Without brake assist, you are unable to flex the system enough to generate a low pedal.

The test for internal master cylinder leakage is to lightly and slowly press the brake pedal and see if it sinks all the way to the floor. It also helps to modulate the pedal , down 1/4" up 1/8" down 1/4" when testing for a internally leaking MC. Bonus feature, drive on a slightly rough road, rest the foot on the pedal, with a bad MC you will feel the pedal fall away as you follow it down.

If the pedal is pressed rapidly, the MC cups flare out due to pressure , seal against the bore and pedal hold height.

Engine off, pedal doesn't go to floor.

The sensors are disconnected at the moment, but I'll connect them soon.


I will definitely order these parts with next day air.

[Father Of Giants]

Originally Posted by Junkman

If you're getting a new ATE master 004-430-75-01-M4 I would bench bleed it by hand, install, connect my bleeder and pull the trigger. There is very little reason to mess around testing the new ATE master. I did have one that seemed to let thee pedal go to the ground. It was/is perfect and started behaving normally once the engine started and wheels were on the ground. Don't make work.

I have an extra cap to the brake master that I use with the bleeder. It will even set the correct fluid level. The inlet sticks down to the correct level. All that is necessary is to bleed, then release pressure , then pull the trigger and siphon the extra fluid back out.

How do I bench bleed?

[Mxfrank]

First thing I would do is to throw away the power bleeder. These toys just seem to make trouble. Bleed the brakes again WITH THE ENGINE ON. As you bleed, make sure the reservoir is kept filled to the top. Start with the right rear, then left rear, right front and finish left front. Then let's see what you have.