Testing the Servo Booster

redrover

redrover

Well-Known Member
Hi All,

I'm not sure if my remote servo is working properly. I've read a few posts on the forum about pumping the pedal with the engine on/off etc, but am not completely sure whether the servo works. How can I test it without removing it from the system?

I've just redone the master cyl and all of the calipers with new seals and hoses etc. I dismantled the servo, but all of the rubbers were fine, so I cleaned it all thoroughly (there were gunky deposits everywhere) and reassembled it.

With the engine off, the pedal is now rock hard (as it should be). But with the engine running there is now significantly more travel on the pedal than before (about 3/4 of its total movement). I've had a little test drive and have found that I need to push the pedal a lot further to achieve braking, but don't have to push it anywhere near as hard as I was previously used to. With the engine stopped, I can expel the air from the servo with 2-3 pumps of the pedal, and it's rock hard again.

Does this mean the servo is working? I was used to a very firm pedal before that only moved about 1/2 of its total travel, but I'm beginning to think that maybe the servo wasn't working before, and is now.
All I know is that the brakes feel very different, and I don't whether that's a good think or a bad thing!!

Cheers all
Michael
 
From what you say the servo seems fine.
What you describe is probably a pedal with a spongy feel. Did you fit new pads too? These take a while to bed in and until then, you will have some spongy feel.
Also if there is still some air in the system (i.e. it needs more bleeding) you will feel that the pedal is spongy too.
 
Hello Michael,

The pedal travel that you have seems excessive for normal braking, so I would be inclined to say that you have air still in the system. When you bleed the brakes, start at the rear, then n/s front and finally o/s front.

I have never experienced a spongy pedal after fitting new brake pads, not saying it cannot happen, just that it has never happeded to me in 26 years of driving a P6B.

Oh and on reusing the old seals from the servo,..it is far better to fit new ones and to be sure rather than have to pull it apart again any time soon. Monitor the brake fluid level for the next few hundred miles and see if it drops at all.

The last time my booster (servo) was overhauled I did it myself, but on prior occasions I relied on a brake business to do the work. On all of these occasions when I relied on the so called experts, the pedal felt odd and the brakes did not feel right for quite some time afterwards.

Ron.
 
SydneyRoverP6B said:
I have never experienced a spongy pedal after fitting new brake pads, not saying it cannot happen, just that it has never happeded to me in 26 years of driving a P6B.
Click to expand...

Or any other car, come to that.
 
Another possible option is that the rear brake pads aren't tight up to the disc. You ought to be able to tell that from a very long handbrake.

Chris
 
Hi Guys,

Thanks for your replies. I checked over the whole system again yesterday and everything seems to be fine with the hydraulics. The pedal is rock hard with the engine off (even after expelling the servo after a short town drive) so I'm confident there's no air in the system.

After a little drive this morning on the way to the MOT centre, I noticed that although there is a lot less pedal pressure than before, I don't think it moves any further (must have been my imagination as it just felt so different!). Furthermore, I can feel the pads gripping at half pressure, but the car doesn't brake as hard until you push further. Thinking about it, this may be down to the new pads (which are new all round), which was something I didn't consider before.

Thanks for the advice!

On another note, I found this article on an MGB site that explains how to test the servo operation. It helped a lot, and proves that my servo is working:
The test is in the second paragraph beginning "For the purposes of the UK MOT..."
http://www.mgb-stuff.org.uk/braketext.htm#servo/
There's also a nice diagramatic explanation of its operation, which always makes for good bedtime reading!
http://www.mgb-stuff.org.uk/servo.htm

Fingers crossed for the MOT anyway. It's been a long time coming!

Will keep you posted.
Michael
 
When I was a callow youth, I used to use DS11 pads (remember them?). The York approved way of bedding them in was to drive to a handy 1 in 4 hill nearby and do emergency stops until the discs were glowing red. They worked beautifully after that!

Chris
 
SydneyRoverP6B said:
DaveHerns wrote,... With new discs and pads, that would appear to be normal,..according to this article...http://www.stoptech.com/tech_info/wp_bedintheory.shtml
Is that terrible in the case of spongy or terrible in the case of they just didn't pull the car up as well as they would once bedded in?

Ron.
Click to expand...

This may be the answer- I've fitted new pads and NOS discs all round, so to all intents and purpose, the brakes are brand new. I took the car on a 50+ mile drive today - mixture of town, country and motorway driving - and had a good few opportunities to brake harshly with nobody around. At the end of the day, the brakes were significantly firmer than before, and more like normal. They're still not exactly as you'd expect, but evidence that the pads are bedding into the new discs nicely. Another few hundred miles and they should be fine.

I'll let you know.
Michael
 
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