Brake servo

[sujitroy]

Is there any way to test a brake servo while it's off the car? I'm suspecting the one of the car is failing and I'd like swap it out with a spare. With the current one I have assist and then no assist and now I can here a hissing sound, in the foot well, when I step on the pedal.
Sujit

[sujitroy]

Got an answer from another forum. Plug the hole where the master cylinder fits and then apply a vacuum with a pump. If it holds vacuum for 30 secs. it's good.
Sujit

[KOY 23]

Did they say what vacuum should be applied?

[sujitroy]

15inHg. Can do the test with the servo in the car as well like this.

[richardthestag]

Usually plug the holes where the master goes and then my finger over the vacuum hole. press the push rod and see if it holds for at least 15 seconds.

Like the more accurate approach though, thanks

[sujitroy]

I looked into this over the weekend, I blocked the hole in the spare servo and connected my Mityvac pump to the valve. I pumped it a few times and nothing. I covered the opening with my hand as much as I could and I couldn't feel a vacuum. I then did this experiment on the servo on Stag no.2 car. Not much vacuum there either. I reconnected the manifold vacuum to the servo and started the car. I disconnect the manifold vacuum and reconnected my Mityvac. I now have a vacuum reading of about 10 inHg, but not much more. It does hold vacuum for more than 30 seconds. Having to do it this way, I can only assume I'd have to pump the Mityvac many times before I could get a reading.

I did this same experiment on Stag no.1 and had to follow the same procedure and results. Stag no. 1's brakes are OK.

I need to do some more experiments,, but I'm thinking at rest the servo holds vacuum, but when I press on the brake pedal it leaks. I will try and add an inline vacuum gauge with manifold vacuum and press on the brake pedal and see if there is any difference.

Sujit

[richardthestag]

press the brake pedal hard, start the engine and the pedal should sink just a little.. Indicates that the servo is working.

However if you have doubts about the servo get someone who know about them to have a looksie. Experimenting with a braking system and advertising it on an open forum when you live if the most litigious county on the face of the planet .. just LOL

[sujitroy]

press the brake pedal hard, start the engine and the pedal should sink just a little.. Indicates that the servo is working.

However if you have doubts about the servo get someone who know about them to have a looksie. Experimenting with a braking system and advertising it on an open forum when you live if the most litigious county on the face of the planet .. just LOL

Hello Richard,
The problem is I have assist most of the time. I'm not losing brake fluid through it. The car was sitting idle for many years and its possible it could be rusty inside. In any case, I'm not going to put the spare one on the car as that's been sitting idle for years too. A repair kit costs about $100, but a rebuild one goes for about 250$. I'll likely have it rebuilt.
I do plan to open up the spare one and take a look inside.

If anyone is interested, here's a document someone created on how to rebuild a TR6 booster.

https://static1.squarespace.com/static/5c6dec53b10f25d4edf0b3f7/t/5d019273143f4a00012250c9/1560384116956/Overhauling+Brake+Servo.pdf

Regards, Sujit

[KOY 23]

In the meantime let’s hope that the servo is working when someone steps out in front of you.

[sujitroy]

I'm not driving it much, but when I do I am maintaining more that double the braking distance. Actually, I shouldn't say I don't have assist. as it would be near impossible to stop without assist. I have instances of less assist.
Sujit

[DJT]

I'm not driving it much, but when I do I am maintaining more that double the braking distance. Actually, I shouldn't say I don't have assist. as it would be near impossible to stop without assist. I have instances of less assist.
Sujit

The servo on mine packed up a few years ago. Had to drive 50miles home from North Devon down the A377. Even leaving 3 times the normal braking distance between me and the vehicle in front it was quite stressful. Brakes still worked, but required a lot of effort.

[sujitroy]

I'd prefer the servo to pack up completely than behave like this.

[trunt]

The servo on mine packed up a few years ago. Had to drive 50miles home from North Devon down the A377. Even leaving 3 times the normal braking distance between me and the vehicle in front it was quite stressful. Brakes still worked, but required a lot of effort.

Haha

I have bad dreams about that.. I always manage to stop .. JUST.. then wake up in a sweat…

[022Dave]

Haha

I have bad dreams about that.. I always manage to stop .. JUST.. then wake up in a sweat…

Yes, but Sujit is doing it for real & knowingly. He's a braver man than me!

It's one thing if the servo packs up whilst out driving (as DJT), but an entirely different matter to take out the car knowing that it has faulty brakes.

Dave

[sujitroy]

Just an update. I had to move the car last night and noticed as I press on the brake pedal, I can hear an air leak under the pedal area. When fully pressed no air leak and when at rest no air leak. This could explain why I was experiencing less assist. Anyway. the booster's going to be sent out for a rebuild.
Sujit