Lancia Motor Club

Model Technical and Interest => Fulvia => Topic started by: Beckerman67 on 03 December, 2021, 08:23:09 AM



Title: Brake Servo Adjustment
Post by: Beckerman67 on 03 December, 2021, 08:23:09 AM
 I have a 1972 Fulvia 1.3S S2. The car has had a history of binding front brakes. I have recently had new front discs and pads fitted. The old ones had become so hot over the years that the discs had turned blue, the pads had cracked and the rubber bleed nipple dust covers melted.
 I am about to change the brake servo, however, I cannot find the gap specification between the servo pushrod end and the primary piston in the master cylinder. Can anybody help? It seems probable that this has been the cause of the overheating brakes due to the pushrod having a negative clearance and constantly engaging the primary piston.


Title: Re: Brake Servo Adjustment
Post by: lancialulu on 03 December, 2021, 11:53:51 AM
According to the drawing in the concise manual the domed nut that pushes the master cylinder piston should be flush with the front face of the servo.

I have suggested in the past that if the brakes bind like you have indicated then undoing the master cylinder a bit an packing with washers to see if the servo is at fault.


Title: Re: Brake Servo Adjustment
Post by: nistri on 03 December, 2021, 07:53:43 PM
Unlikely that the problem is in the servo, more likely there is a problem with the master cylinder, Andrea


Title: Re: Brake Servo Adjustment
Post by: Beckerman67 on 04 December, 2021, 04:25:15 PM
 When everything becomes hot the brakes start to bind. If I undo the two nuts that connect the master cylinder to the servo unit the pressure is released and the binding stops. This tends to suggest that the master cylinder pistons are moving OK and that the pushrod is protruding too far out and engaging with the piston.
 The servo appears to have been rebuilt a number of times over the years according to the paperwork history so I'm going to start with a new one ensuring that the pushrod gap is acceptable using a vernier caliper.


Title: Re: Brake Servo Adjustment
Post by: nistri on 04 December, 2021, 06:21:29 PM
It is simple to check if the servo is responsible for the symptom. Its hose from the intake manifold can be disconnected and carefully plugged so that the engine can run OK.  If the symptom persists, then it is not a fault of the servo, Andrea


Title: Re: Brake Servo Adjustment
Post by: davidwheeler on 04 December, 2021, 08:04:37 PM
I agree with your diagnosis and have posted on this before.  Not sure of the exact dimension you seek though, I just used trial and error!


Title: Re: Brake Servo Adjustment
Post by: Beckerman67 on 04 December, 2021, 08:58:07 PM
 Really sorry Andrea, but I'm not following that logic.
 If the servo pushrod protrudes out too far from the mating surface with the master cylinder, as soon as you bolt the two together the pushrod will engage with the piston partially applying the brakes. This would be without any assistance from the servo vacuum and so disconnecting the vacuum hose would have no effect.
  


Title: Re: Brake Servo Adjustment
Post by: nistri on 05 December, 2021, 07:33:00 AM
If the pushrod is too long, it is unnecessary to replace the servo: it would be enough to adjust its length as suggested earlier. To actually produce brake binding and overheating of the discs, it must be off for quite a length and presumably affect all 4 wheels. Advisable to check that the hub overheating has not damaged the wheel bearings, Andrea



Title: Re: Brake Servo Adjustment
Post by: davidwheeler on 08 December, 2021, 09:55:02 AM
Really sorry Andrea, but I'm not following that logic.
 If the servo pushrod protrudes out too far from the mating surface with the master cylinder, as soon as you bolt the two together the pushrod will engage with the piston partially applying the brakes. This would be without any assistance from the servo vacuum and so disconnecting the vacuum hose would have no effect.
  
No, it is necessary to disconnect the brake pipe and release a little fluid.    I used to do this, the brakes started off fine but as soon as they got warm they started to bind.