411 brake problem

[Kevin H]

After several months of not being driven, or even started, my 411 seems to have lost it's brakes.

Other than the return spring there is absolutely no resistance when pressing the brake pedal, even by hand.

It was fine a few months earlier when I last drove the car.

The brake fluid reservoir was quite low, but I cannot see any sign of leakage from either the master or slave cylinders. To be honest I can't remember the last time I checked the brake fluid level so it could have been like that for quite a while.

The only thing I can think of that could explain the break fluid loss is that it's going into one of the vacuum boosters, but I don't think that would explain the zero resistance on the brake pedal, or would it?

I guess I'm going to have to remove the master cylinder to investigate that. Thankfully it's a very easy job in the 411.

How can I tell if one of the boosters have failed without removing it from the car?

Any other suggestions welcome

[GREG]

Maybe try bleeding the brakes first and then start the car with your foot hard on the pedal -- the pedal should drop slightly and this shows the booster is working !
Then look for leaks around wheels, callipers,hoses, pipes etc

I suspect that if you are not seeing leeks anywhere that one of the servo/boosters has failed and the fluid has leaked inside them.

Fit new seals or buy an exchange unit. -- lots of fun :-)

[Kevin H]

Originally Posted by GREG

Maybe try bleeding the brakes first

Thanks. I hate working on brakes

Is there any particular sequence they should be bled in?

[GREG]

Top up the master cylinder reservoir with new fluid.

With a suitable lift or jack, raise the car to gain access to the rear suspension underside. Ensure that the car is sufficiently secured on jack stands or ramps, and chock the wheels before getting under it.

Slide under the car and look at the rear calipers. These are mounted inboard of the wheel hubs. Toward the top of each caliper, you will find its bleeder screw. Begin with the bleeder which is farthest from the master cylinder: typically, the right rear. Fit a rubber vacuum hose onto it and put the other end of the hose into a drain pan to hold the old fluid. It is important that the hose be fully immersed in fluid: put some fresh fluid in your pan to start. Put a spanner on the bleeder and open it about a half turn. The screws are probably rusted, so go easy and use some penetrating oil. Be prepared for a little brake fluid to seep onto your face (you want eye protection and in my opinion, it doesn't taste very good). Now have an assistant push the brake pedal smoothly all the way to the floor and hold it there. This will force old, contaminated fluid out through the hollow bleeder screw and through your hose into the drain pan. Is the fluid brown? It's supposed to be clear! Finally, tighten up the bleeder. It is important that the assistant understands not to release the pedal until you have tightened the bleeder screw. (If the pedal is released, the system will suck air into the line through the open bleeder screw. Keeping the end of the hose immersed in fluid will reduce the chance of air bleeding back into the system. Brakes don't work with air in the lines. You will simply compress the air and the pads won't move against the discs.) Now, with the bleeder screw closed, have your assistant release the pedal. Repeat this opening bleeder - pumping pedal - closing bleeder - releasing pedal routine several times until the fluid coming out of the hose is clean.

Now, move on to the left rear and repeat the process there, continue on to the front right and front left brakes in that order. Throughout the job, keep the master cylinder reservoir topped up. If it empties, air can be introduced to the system here as well.

Now your brake pedal should be firm and strong with no sponginess. Or now look for leaks :-)

[UHT405]

One quick check would be to remove the vacuum hose from the brake servo and examine inside the end. It should be dry. If it's oily with brake fluid - be suspicious of a leak in thec servo!

[Kevin H]

Originally Posted by GREG

Or now look for leaks :-)

Thanks Greg. I've already looked for leaks and everything visible externally is dry as a bone - master cylinder, calipers, pipe joints.

Before I try bleeding the brakes I think I need to investigate why there is no resistance to the brake pedal, because I'll be wasting my time if the master cylinder is kaput.

I'll check out the vacuum tubes on the brake boosters too given that's a five minute job. Thanks for the tip UHT405!

[Stefan]

Sounds like the master cylinder washer has gone floppy. Simply bleeding the brakes thoroughly might actually restore the pressure in your system - I've seen that work for years and years and years - but Id feel happier with a rebuild of the MC in your heavy, fast car...

[MTH 411]

I suspect you will find the leak into one of the servos. This are common parts and you can source a refurbished item fairly readily

Mike