No brakes on pug 205

mote · Post by **mote** » 01 Oct 2004, 00:47

I have a pug 205.
With the engine off the brakes feel OK.
With the engine on the pedal hits the floor real easy without braking the car.
Neither brake circuit works.
If I pull servo hose off the vacuum pump to disable the servo then the car does brake under foot pressure alone. Obviously not very quickly.
The fluid is not dropping and there are no visable leaks.
I can hear a distinct hiss inside the car when I depress the brake pedal, engine running or not.
I tried changing the master cylinder for one from the scrappers but it made no difference. It was a scrapper so could have been coincidentally faulty.
Put the old cylinder back and got a refund.
They didn't have another.
Could the servo or some other fault case this problem?
Should I try another master cylinder?
A new servo?
Any suggestions?

Stuart McB · Post by **Stuart McB** » 01 Oct 2004, 01:07

If it's a diesel then there is a one way valve on the servo unit. If this valve breaks it allows no vacume for the servo to be kept in the servo unit. If petrol it again sounds like a servo problem as you say you have brakes when it's disconnected.

mote · Post by **mote** » 01 Oct 2004, 04:56

Thanks for the reply Stuart
It is a diesel.
I did the test where you push the pdeal down with the engine off and then turn the engine on. The pedal went down further as it should. Likewise after I turn the engine off it takes a few pumps of the pedal to exhuast the vacuum and bring the pedal back to the top.
If the one way valve broke what would be the effect? Would that not be the same as not having the servo connected?
I don't know the exact details of what goes on in a servo so it is hard for me to visualise what might be happening.

Stuart McB · Post by **Stuart McB** » 01 Oct 2004, 20:41

Before getting your wallet out just make sure that the front calipers haven't stuck on and that the rear shoes ajusters are OK. If the calapiers have stuck while pressed out you will have a sort of spongey feel to the brakes and pumping is required same as the rear.

mote · Post by **mote** » 01 Oct 2004, 21:53

It's not that. I have done all four brakes recently and they are all Ok. I have checked them since the problem started.
It is also definitely related to servo use.
I can imagine it is the master cylinder seals and the servo provides enough umph to push the fluid past the seals where foot alone does not.
But my gut reaction is the servo itself.
I need a better understanding of what goes on inside it to visualise what might be happening.
The brakes don't go spongy the pedal just goes to the floor. Likewise the brakes are solid without the servo.

Stuart McB · Post by **Stuart McB** » 01 Oct 2004, 22:06

Got to say then this sounds just like the servo. My old ZX td had the same. Servo sorted the problem straight away. I know that it's better to just fit a new one as they are a sealed unit / part. All that happens is that a vacume is made in the unit by manifold pressure or in the case of a diesel a vacume pump, this is then used with a diaphram to boost the pressure so that it can apply the brakes (in this case). The push rod on the brake pedal is attached to the servo unit. In short small pressure made bigger. Small amount of fluid made bigger with use of vacume prssure. It's not a big job to remove and fit a new unit, just a lot of looking up from the drivers foot well.

mote · Post by **mote** » 03 Oct 2004, 19:23

Interesting that you had the same problem and it was the servo.
I agree it is better to change the sealed unit.
What I don't understand is how the servo can have this effect.
It does seem to get and retain vacuum.
I could understand if it were leaking and not giving as much assist as usual.
But how does it just lose all braking when the vacuum is applied?
What can stick/leak/whatever inside to cause this effect and how does it happen?
I just like to understand things and cannot understand how the linkage is working without vacuum and vanishing with vacuum.
If I do swop it out and it works I will cut the old one open for a look.