Help with lack of progression in brakes

[sabbot]

The brakes on my elan +2 are great but not progressive. It is fitted with 2 rebuilt Girling mk2b Servo units. The problem I have is that the brakes work but the retardation isn’t proportional to the pedal pressure. Gentle pedal pressure slows the car slightly and then as I gently press the pedal further all of a sudden the brakes come on quite hard. I’ve checked for vacuum leaks, taken the servos apart to check for fluid leaks and sticking pistons and can’t find anything wrong. Any and all suggestions would be appreciated. - thanks

[oldelanman]

Have you replaced brake pads or seals recently ? If the caliper pistons are pushed back fully during the process it can take a while before pad/disc clearance returns to normal (due to seal rollback effect I believe) and this can give rise to similar symptoms to what you describe, albeit more a delay than a lack of progression. I had this recently after changing front caliper seals and eventually overcame the problem by the old trick of wedging the brake pedal down overnight to pressurise the system, the excessive clearance disappeared.

[2cams70]

What size is the bore in the master cylinder - is it standard? Are your brake calipers standard?

[sabbot]

@oldelanman : The whole brake system was rebuilt, new seals etc. I haven’t double checked the rear calipers but I’ve checked the fronts and they are fine.

@2cams70 : I was also wondering if this is a master cylinder issue. The master cylinder is a standard federal tandem unit (with new seals). I don’t think it should make a difference but the front brakes are fed from the rear (closest to bulkhead) output port. And each half of the tandem master runs through the pressure differential warning switch unit before going to each servo.

The whole brake system has been bled and the pedal is solid, just unfortunately doesn’t lead to progressive braking.

Thanks for suggestions and I will certainly appreciate any others.

[Billmack]

I am not a fan of those servos and I think it is there you should look for the problem. I have noticed a time delay between when you push the pedal and when you get full boost. Possibly try new ones if the latest design. I have rebuilt quite a few girling servos with so so results at best. Expensive kits and often the body of the servo is corroded beyond repair..

[reb53]

I had the professionals rebuild my servo even though my own rebuild had been fine for 30 years.
The returned servo exhibited similar problems to yours, and after another attempt by them to fix it, still more.
They returned my money and I bought a replacement, new, Lockheed.
No problems now.
But I do have a nice looking, "re-built" Girling servo, that doesn't actually work.......

Ralph.