Spongey brakes - master cylinder fix

[Station]

Nearly every car I've had has done this - the brake pedal is spongey, and no matter how many times you bleed it, it's the same, or worse.

I've seen this a few times before on here, and thought I'd post these up.

 

Over time, the bore in the master cylinder will get clogged because of particles in the reservoir. If you look into the bottom, it'll probably be black. This is all going into the cylinder. I don't know where all this black stuff is from - possibly fluid degrading/heat/dust. Either way, it'll eventually be bad for the braking system.

 

My MR2 brakes went spongey the other week, there was only slight resistance with the pedal on the floor.

 

These things were sat on the rubber seals on the piston:

 

 

They're hardened black buildup that let fluid in and out, when it should only be going in.

 

The cylinder unbolts from the servo - undo a bleed nipple on the caliper and force all the fluid out. Take all the unions off the cylinder, and then unbolt the cylinder from the servo. The piston is held in with a circlip on the back when the pedal pushes it in. On modern-er cars, there are two pistons for each circuit - one is held in place with a small bolt in the size.

 

Knocking the cylinder will make the pistons come out. Clean the whole the out with hot soapy water, then dry them properly, and then put new brake fluid in. Brake fluid is water soluable, so you don't need solvents, etc, it's the best way to clean it out.

The braking resistance will be much much better everytime guaranteed.

 

Look at all the crud that came out of the system:

 

 

This whole fix took about 30 minutes, and that was mostly bleeding the brakes again.

[Station]

Here's a Mitsubishi clutch cylinder straight off the car:

 

 

The black is decades of build up!

 

Not a great pic, but once again, the cylinder is full of black gloop and the seals weren't sealing at all. It's not just a little but, the whole thing is lined with gloop - the pedal was weak, and had to go down halfway before it felt anything. Cleaning it out made it feel completely different.

 

 

Cleaned out with hot water/washing up liquid/toothbrush:

 

 

 

Don't used recycled old, brown brake fluid that you've just pushed out of the bleed nipple; use new stuff, it's only cheap!

 

[Rusty_Rocket]

All good but the black tends to be the worn remnants of the rubber seals! We have a lovely little hone for brake cylinders at work- it can rescue a few cylinders but if there's any pitting it's a pointless task as the pressure just leaks past.