Coming back from brake failure - how we did it

After the rather dramatic end to our summer holiday, Husband was faced with getting the van back on the road sharpish in time for a weekend away - here's his story on how he did it:

So what was the symptom with this brake failure? The brake pedal went all the way to the floor with no braking effect, and there was no visible sign of leaking fluid from any of the wheel cylinders - this lead me to suspect the seals in the master cylinder had gone.

Here's a brief bit of my theory: you press the brake pedal, the servo amplifies the effort using a vacuum to suck the pedal on, and the pedal is directly linked to a plunger in the master cylinder. The plunger pushes brake fluid down the brake pipes, and at the wheel an opposite plunger is pushed out moving the brake shoes or pads to touch the brake drum/disc.

So as reported, having done an emergency stop a while earlier, suddenly the pedal started travelling much too far without any braking effect. Eventually it completely failed – the seal in the cylinder must be completely gone. So to fix it, first I drained the fluid by opening the bleed valve at the front wheel. That allowed all the brake fluid in the tank to run through. Once it was empty I undid the nut securing the brake pipe to the master cylinder. Then the two bolts that retain the master cylinder to the servo (no need to even remove the pin from the brake pedal) and the master cylinder can be removed. Now remove the circlip from the end of the cylinder:

Removing the circlip

The reservoir can be removed by undoing the nut inside. It's easier to hold in the vice once this is off. I also found the seal under the reservoir was in very bad nick so it needed changing as well. 

The old seal is that mess stuck to the underside - new seal slightly different

The circlip and washer behind it were a bit rusty, but cleaned up OK. I then had to push the plunger down in the cylinder using one screw driver to allow a ring of rusty crap to be scraped out using another one. Once that was cleaned up, with a bang on the bench the innards of the master cylinder fell out. The old seal was visibly deformed but not split. Compared to the new one it was clear that it wasn't sealing properly though. There are two seals, so both have to be removed. The tough one is fitted on the plunger, and has to be removed with a small screw driver to lever it off. Re-fitting is opposite being careful not to damage the new seal.

new seal on the plunger

second seal located in the top of the spring

Now just put it all back together. Put the spring in first with the second seal loosely located in the top. Then the plunger. Now the tricky part – pushing the plunger down while trying to get the circlip on. Once done though, it's back together.

Pushing the plunger back into the cylinder

Now fit the dust seal on the end of the master cylinder, and then bolt it all back onto the servo

Dust seal fitted

Now clean off the top face of the master cylinder where the reservoir is going to fit and bolt the reservoir back on.

Cleaned up top face - be careful not to lose the steel collar that goes around the weld bolt

New seal fitted

And finally, fit a new seal in the top of the reservoir cap.

Reservoir seal cap

Now re-bleed the brakes and you're away. I used an eezi-bleed kit and it worked really well (although i've had mixed results in the past with them and usually prefer gravity bleeding or two man pedal pushing/bleed valve turning). I only bled the front right wheel, this was the one I'd allowed the fluid to run through before, and it turned out the rest didn't even need bleeding. The pedal feel is much more responsive now, and the brakes are definitely a bit sharper. 

Job's a good un, now to suss out the dodgy starter that I'm still being plagued by....