Technical Uno Brake Problem

[Johannes Hideous]

My Uno 1993 Special Edition 1100 has been giving me an intermittent problem. When vehicle is travelling on a slight down gradient the brake pedal sometimes goes down to the floor. I have renewed the master cylinder as well as the front calipers and rubber hoses. The system has been bled numerous times and the brake fluid completely renewed. When the pedal goes down to the floor, it sounds like it is sucking air when it comes back up and then the brakes are fine for a couple of days. Any ideas on where else to look for this problem? The car has been to the Agents workshop 5 times now with no success in remedying the problem.:bang:

 

[alexGS]

Hi Johannes, and welcome to the forum

What a horrid fault, intermittent or not. I don't think anyone would enjoy the sensation of the brake pedal sinking to the floor when driving down a hill.

It seems that you have covered the most likely possibility (the master cylinder), though it is just possible for two master cylinders to be faulty in similar ways. I think the key would be that you would expect at least some change after replacing the master cylinder.

One thing you didn't mention is the brake servo. This is powered by engine vacuum, which is at its highest with the engine over-running, going down a hill. There should still be a mechanical connection between the pedal and the master cylinder, so I'm not certain this would allow the pedal to go down even with the servo faulty. But also the sucking sound (when the pedal comes back up) seems relevant for something that has engine vacuum on it.

You may find the brake servo to be full of brake fluid, which seems to happen when the master cylinder pushrod seal leaks. Perhaps this is remaining from a previous master cylinder problem, and causing problems in the servo.

Brake servos are not a common failure (all things considered) so I would suggest trying a replacement from a wrecker's yard. On the other hand, some people refuse to use safety-related parts from wrecked cars, so it's up to you (but it's certainly not very safe at the moment is it! )

-Alex

 

[phil_harry]

I had this problem with my uno on the M25, a very scary moment!

It turned out to be the master cylinder at fault, but with major rust problems dogging the 120,000 mile car, I had to make the difficult decision to scrap her, so I never had chance to have a go at changing it. sob

Phil

 

[rawill]

Yes Master Cylinder ???

I had this prob in an Alfa once. I would double check the master cylinder and servo.