Air in Master Cylinder Or Damage?

[EricSilver]

Car: Any W124

If you push back a brake piston while the master cylinder reservoir cap is still on what happens?

WIll it damage the MC? If you then remove the cap and pump the brakes, will that suck air into the system?

[Duke2.6]

No, but why not do it the easy way. Use a box wrench to loosen the bleeder screw to just snug, then attach a bleed hose to the nipple and route it to a container. Open the bleeder valve, push the piston back with your thumbs, then close the valve. This will purge that old fluid from the caliper and once you finish with the pad change you can do a complete system flush.

If you try to push the piston back with the bleeder screw closed the only path out for the fluid is through the tiny compensating port in the master cylinder bore. You'll need a screw clamp to push back the piston, and if the reservoir is near full the extra fluid can overflow the vent hole.

Duke

[EricSilver]

Hi Duke,

The reason I asked is that I was checking my pads and pushed back the piston with a C-Clamp. Shortly thereafter I realized the reservoir cap was still on, so I removed it. I replaced the cap when done, and pumped the brakes.

When the car engine is off, the pedal is rock solid. When the engine is running, the pedal is spongy. I assume air is the culprit?

[Duke2.6]

Your procedure should not have introduced any air into the system. The reservoir cap is vented so the fluid pushed into the cylinder will just displace air.

With the engine off if you pump the brakes two or three times to deplete the vacuum reservoir the pedal will feel hard. Start up the engine and press the pedal and it will feel spongy because the vacuum boost allows the pedal to go down farther with the same force. This may be what you are experiencing.

The true test is whether the brakes perform as before on the road. It's not clear to me why you folded back the caliper (front) to check the pads. They can be easily checked with the wheel off the car through the caliper window and you can also see the ends of the pads from the front and rear of the caliper.

The important point on the rear brakes is to remove and push back the piston ONE SIDE AT A TIME. Since the rear are fixed calipers wiht pistons on both sides, if you remove both pads and push on a piston the other piston will pop out and can also pop out even if you open the valve.

Duke