Testing a brake servo

MartinClan · #1 19th March 2009, 07:53

Like most of us I have a second hand brake servo and I am beginning to get a bit suspicious of its integrity. Yesterday, when driving up and down the drive, I noticed an occasional weazy squeel when applying the brakes hard, followed by the engine hunting for a few seconds. I guess there must be a vacuum leak somewhere. But how do I tell if it is the servo itself - or just a leak between the master cylinder and the servo?

Robin

Patrick · #2 19th March 2009, 08:14

I had a leak between the servo and master cylinder, and then also on the pipe connection to the server. I found it my holding my hand over the area's and also listening closely (not so easy as the M20 is a bit of a loud contraption!)

peterux · #3 19th March 2009, 09:45

That may be normal operation?

The standard test is to switch off the engine and apply the brakes a number of times to get rid of the vacuum. Then press the brake pedal hard and start the engine. After you start the engine and the vacuuum builds up you should feel the pedal move down a bit.

However, the one area you might have a slight leak is where the vacuum connector goes in a rubber grommet into the servo body. Mine was a bit loose so i coated it with a bit of black silicon and pooped it back in.

Did you use any silicon sealer when you assembled the Marlin adapter and brake master cylinder?

MartinClan · #4 23rd March 2009, 07:48

I am still not happy with my servo even though it passed the MOT. I would really like to source a new one but Metro ones seem impossible to get at a sensible price. Has anyone else considered an alternative? The mini one looks very similar and has the added advantage of an adjustable push rod.

But would it fit?

Robin

Patrick · #5 23rd March 2009, 08:20

Depends if the holes line up, size wise it looks OK. No reason you couldn't make a new plate to go on the end of the pedal box if it does not fit.

GreatOldOne · #6 23rd March 2009, 12:00

To be honest, it looks exactly like the one that Marlin supplied me way back when, albiet in black:

MartinClan · #7 23rd March 2009, 12:16

Quote:

Originally Posted by GreatOldOne

To be honest, it looks exactly like the one that Marlin supplied me way back when, albiet in black:

Thanks G.O.O. Can you remember if it had an adjustable pushrod? The Metro one I have used (which also looks very similar) has a fixed length pushrod for which Marlin supplied a 'orrible adapter.

Cheers

Robin

peterux · #8 23rd March 2009, 13:17

The metro would have been a development from the mini one. Maybe the fixed length pushrod was just a cost saving? It certainly looks similar. I managed to pick up a 'new old stock' metro one off eBay but can't remember what I paid.

i meant to add to try these folks as they are normall very reasonable prices ...

http://www.mpdonline.co.uk/PhoneNumbers.htm

GreatOldOne · #9 23rd March 2009, 14:49

I don't believe it had an adjustable pushrod - I had to use the Marlin adaptor. Probably means that it's a Metro servo...

I'll take a peek later when I get back from work. I think there is a part number stamped on the side as well, so I'll jot that down.

GreatOldOne · #10 23rd March 2009, 15:43

I can't really see if it's adjustable as the pedal box is in the way, but I've just looked and I definitely used the Marlin adaptor.

The numbers on the plate on the side of the Servo are:

L 41 A 6

LBDHZCO14A4 (that O could be a 0)

MartinClan · #11 23rd March 2009, 16:00

Quote:

Originally Posted by GreatOldOne

LBDHZCO14A4 (that O could be a 0)

That's the same as on mine. (I happened to have a note of it in my Marlin notebook). So I guess its a Metro one.

Hmmm.

I shall have to look into this further......

G.O.O. I suspect the reason yours is black is the same that mine is. It was looking a bit past it's best at 10 years + old with the plating very tired...

Robin

Mike · #12 23rd March 2009, 20:08

Quote:

Originally Posted by MartinClan

That's the same as on mine. (I happened to have a note of it in my Marlin notebook). So I guess its a Metro one.

Hmmm.

I shall have to look into this further......

G.O.O. I suspect the reason yours is black is the same that mine is. It was looking a bit past it's best at 10 years + old with the plating very tired...

Robin

Robin

I have been round several Metros in the breakers yard & I found 50% had black servos and the rest were poorly galvanised, so I chose a black one.
There really is very little to go wrong with a servo. They either work or they leak. I bet yours is working just fine. Rember the brake boost on it is about 2:1, whereas on modern cars the boost is much higher.
I have looked at using a Mark IV Cortina servo, as that allows the use of a standard Sierra master cylinder, without having to make an adaptor plate. It would fit with your M20 in the Roadster, but will not fit with my M50 in a Cabrio, so Ive stayed with the Metro servo.
An other option is to go for a remote servo such as is fitted to the MGB.......?
Mike

Patrick · #13 23rd March 2009, 20:29

The 5exi used a remote servo does it not, I'm sure the builders would be able to advice on that front

MartinClan · #14 24th March 2009, 08:13

Quote:

Originally Posted by Mike

I bet yours is working just fine. Rember the brake boost on it is about 2:1, whereas on modern cars the boost is much higher.
I have looked at using a Mark IV Cortina servo, as that allows the use of a standard Sierra master cylinder, without having to make an adaptor plate. It would fit with your M20 in the Roadster, but will not fit with my M50 in a Cabrio, so Ive stayed with the Metro servo.Mike

Thanks Mike

I am actually using the standard BMW master cylinder so the Cortina servo probably is not an option. I would be interested to know where the boost ratio of 2:1 comes from. Can you work it out or is it just something that you happen to know? I guess the main factor is the size of the diaphram (and hence the size of the servo). As you say it isn't a lot.....

Kind regards

Robin

Mike · #15 24th March 2009, 09:49

Quote:

Originally Posted by MartinClan

Thanks Mike

I am actually using the standard BMW master cylinder so the Cortina servo probably is not an option. I would be interested to know where the boost ratio of 2:1 comes from. Can you work it out or is it just something that you happen to know? I guess the main factor is the size of the diaphram (and hence the size of the servo). As you say it isn't a lot.....

Kind regards

Robin

Robin
Does your standard BMW master cylinder only have two outlets?
I think this is something which will niot be allowed under IVA - good move getting in under SVA!!
Regards
Mike

Mike · #16 24th March 2009, 09:58

Quote:

Originally Posted by MartinClan

Thanks Mike

I am actually using the standard BMW master cylinder so the Cortina servo probably is not an option. I would be interested to know where the boost ratio of 2:1 comes from. Can you work it out or is it just something that you happen to know? I guess the main factor is the size of the diaphram (and hence the size of the servo). As you say it isn't a lot.....

Kind regards

Robin

Robin
I just Googled Servo brake bosst ratios and found this.......
https://www.minimaniauk.co.uk/web/th.../msgthread.cfm
Mike

Mike · #17 24th March 2009, 10:26

Quote:

Originally Posted by Mike

Robin
I just Googled Servo brake bosst ratios and found this.......
https://www.minimaniauk.co.uk/web/th.../msgthread.cfm
Mike

Robin

I 've just found my Audi A4 has a boost ratio of 5.5:1, so you can see the difference with only using a Metro one at around 2:1

Mike

MartinClan · #18 24th March 2009, 11:33

Quote:

Originally Posted by Mike

Robin
Does your standard BMW master cylinder only have two outlets?
I think this is something which will niot be allowed under IVA - good move getting in under SVA!!
Regards
Mike

??? I thought the SVA and IVA were basically the same for brakes - is that not so then?

Oh - and yes the BMW one just has two outlets.

Regards

Robin

limpabit · #19 24th March 2009, 11:53

Quote:

Originally Posted by Patrick

The 5exi used a remote servo does it not, I'm sure the builders would be able to advice on that front

No, does not use a remote servo. Uses one from a bubble 200 Rover. But some don't even use a servo. Copied the Lotus Elise as that does not have one either if I'm not mistaken.

Patrick · #20 24th March 2009, 13:29

Quote:

Originally Posted by limpabit

No, does not use a remote servo. Uses one from a bubble 200 Rover. But some don't even use a servo. Copied the Lotus Elise as that does not have one either if I'm not mistaken.

Ah OK, not sure where I got that idea from