New dual circuit boosters now rear brakes sticking

[cdnp6]

I have just replaced both boosters on my federal dual circuit car and bled the brakes as per the manual. The rear brakes are now sticking on. I have released the vacuum hoses with no change in rear brake tightness. Any idea what to try next?

 

[colnerov]

Investigate the air valves on the servo, the white plastic item. There are plenty of threads and posts on here about the question of the number of 'O' rings and the problems it causes.

 

[sdibbers]

Also, check the flex hose at the rear. I’ve seen them swell with age internally and create a sort of one way valve effect doing this.

 

[cobraboy]

As said above, the air reaction piston - a small piston that lives under the plate that the white plastic moulding screws to on Lockheed remote servos is the likely cause, it can be sticky and slow to move. Sometimes these pistons are fitted with two seals, if so these can trap brake fluid in between them and lock the piston up, the solution is to remove one of the seals and lube the remaining seal.

My own Lockheed servo is displaying a 'stepped' feel to the brake application, and also the brakes are hanging on momentarily, this due to a sticky air reaction piston, the problem is finding the reaction piston seal separately from a complete kit.
I have now found a UK supplier who sells the seal on it's own, they are waiting for stock in the next few days.
Hopefully I'll be able to update with some pics in due course.

 

[cobraboy]

Well, the new seals came in, I bagged a couple, one to fit, and one for a spare.
Lucky to find these separate from a full rebuild kit at Past Parts Bury St Edmunds.

 

[cdnp6]

I have pulled both air reaction pistons and lubricated with silicone grease. The pistons in these units is a single seal as in the picture from a previous members post.
I have reassembled the air valves, started the engine and pumped the brakes a few times then shut the car off. I could now roll the car back and forth with no feeling of resistance. Parking on a couple of small hills the car rolls down easily with no resistance and no sounds of friction from rear brakes.
After parking the car I shot a temperature gun at all rotors. The fronts were both 24 degrees, the right rear read 31 degrees and the left rear is at 40 degrees.

 

[cobraboy]

Temps will vary on rears depending on individual park brake adjustment. Seems like you have found your problem and got a positive result.

With the help of a glamorous assistant I have manage to install the new seal without introducing air into the system and the need to bleed, but I think I am going to have to pull my servo and look for corrosion in the main bore, I still have a stepped feel to the pedal.

 

[cdnp6]

I'm hoping so! Thanks for the help!