Brake Servo Issue

[Darren Groves]

The Powertune brake servo on my Spitfire has a sticky piston on the air control valve (see pic), which results in the brakes locking on even without the pedal depressed. Anyone else had this issue and managed to successfully fix it? 

 

[Pete Lewis]

  yes    a stronger spring may help this if the air piston is stiff it makes the brakes hang on 

something rings a distant bell to remove one of the two seals 

make sure the piston pushrod does move easy through the vac chamber hole where the main cylinder is bolted 

 

Pete

[Wagger]

I seem to remember a blocked hole somewhere (with flaking rust) causing this on a MM 15 years ago. Check for debri or something acting as a valve.

It was very annoying, sapping power, ruining economy and boiling the fluid. Took ages to find.

[Darren Groves]

9 hours ago, Wagger said:

I seem to remember a blocked hole somewhere (with flaking rust) causing this on a MM 15 years ago. Check for debri or something acting as a valve.

It was very annoying, sapping power, ruining economy and boiling the fluid. Took ages to find.

That won't be the issue here as it was effectively a new unit, but bought several years ago so no receipt and can't even remember where I bought it....🙄

[Darren Groves]

21 hours ago, Pete Lewis said:

  yes    a stronger spring may help this if the air piston is stiff it makes the brakes hang on 

something rings a distant bell to remove one of the two seals 

make sure the piston pushrod does move easy through the vac chamber hole where the main cylinder is bolted 

 

Pete

When say stronger spring, you mean the main one to the diaphragm? Where would I even get one from as Powertune don't seem to exist anywhere, can't seem to find out who actually makes these units.

I did unclamp the back of the unit, but didn't seem any obvious way to remove the diaphragm so I could look at the pushrod etc. does it just pull out as it didn't feel like it wanted to.

 

 

[PeteH]

By the look of those o-rings, they have gone hard?. You could try replacing them?.

Pete

[Darren Groves]

Just now, PeteH said:

By the look of those o-rings, they have gone hard?. You could try replacing them?.

Pete

First thing I checked, but seem soft and pliable....

[Pete Lewis]

not the main spring  the one under the air valve as from memory thats all that returns the small  hydraulic piston 

any stiffness can cause the brakes to be dull and hang on 

i had one that persisted in hanging on and in the end i just replaced it despite much strip down and checking  i never solved it 

new unit worked so much better  ...

Pete 

 

[dougbgt6]

Yes, on a Lockheed there's a small spring in the air valve mechanism, only a couple of turns. I know this because I took mine apart and lost it!  I got a box of 200 springs off Ebay and modified one to replace.

 

Spring is under the small black rubber bit, accessed through the other side.

 

Doug