Brake servo locking up brakes!

Rebuilding, adjusting, maintaining & alternatives
User avatar
jcrcleeman
.
United Kingdom
Posts: 38
Joined: 1 year ago
Location: East Yorkshire

Contribution Stars

Brake servo locking up brakes!

Post by jcrcleeman »

Having fitted disc brakes to the front of my minor I also fitted a servo. The servo is rated as 1:9 and unfortunately if I press with all but the lightest of pedal pressure the wheels lock up and I "nut" the windscreen!

Obviously this is has to change, but can anyone suggest a suitable less "fierce" servo ratio and recommend a model/supplier.

The brakes without the servo connected are quite OK for myself driving but too much pressure is needed when my wife drives!

(Marina discs fitted).

Thanks,
John



User avatar
BrianHawley
.
Posts: 501
Joined: 5 years ago
Location: Norfolk

Contribution Stars

Re: Brake servo locking up brakes!

Post by BrianHawley »

jcrcleeman wrote:Having fitted disc brakes to the front of my minor I also fitted a servo. The servo is rated as 1:9 and unfortunately if I press with all but the lightest of pedal pressure the wheels lock up and I "nut" the windscreen!

Obviously this is has to change, but can anyone suggest a suitable less "fierce" servo ratio and recommend a model/supplier.

The brakes without the servo connected are quite OK for myself driving but too much pressure is needed when my wife drives!

(Marina discs fitted).

Thanks,
John
Did you do the master cylinder modification that is needed for disc brakes?




Sent from my iPhone using Tapatalk


Brian

"Jodie". '67 Traveller, 1275, discs, van wheels, vert shocks, arb etc.

User avatar
jcrcleeman
.
United Kingdom
Posts: 38
Joined: 1 year ago
Location: East Yorkshire

Contribution Stars

Re: Brake servo locking up brakes!

Post by jcrcleeman »

Yes Brian, and also added an additional brake reservoir . I'm not sure how servos are rated in terms of pressure function, 1: 9 etc, are other values readily available?
John



User avatar
Custard
.
Ireland
Posts: 2952
Joined: 10 years ago
Location: Blackpool

Contribution Stars

The Cliff Lloyd Award

Re: Brake servo locking up brakes!

Post by Custard »

The ones advertised for kit cars are generally in the 2:1 region.

https://www.carbuilder.com/uk/remote-brake-servo

Interested to know where the 9:1 came from it does seem a bit excessive.

I haven't used this site for brake servos as my kit car didn't have one, but they have been ok for fuse boxes, switches and trim.

They do show a smaller sized one and one for dual circuits all are quoted as having a 2:1 boost ratio which in theory should halve the pressure on the pedal for the same braking force whereas yours only needs a ninth!

Sean.



User avatar
Sleeper
.
MMO Button
Posts: 549
Joined: 5 years ago
Location: Manchester U.K.

Re: Brake servo locking up brakes!

Post by Sleeper »

I don't think the servo is 9:1, the highest I've heard of is 4.25:1 , maybe it is 1.9:1 ?

How do you know the ratio ?

John ;-)



User avatar
Custard
.
Ireland
Posts: 2952
Joined: 10 years ago
Location: Blackpool

Contribution Stars

The Cliff Lloyd Award

Re: Brake servo locking up brakes!

Post by Custard »

That would make more sense, but then the grabbing would have to be down to something else.

Those half shims possibly the wrong way up so the leading edge digs in.

Or pads overlapping the shiny portion of the disc and grabbing a rusty bit with more grip.

Sean.



User avatar
jcrcleeman
.
United Kingdom
Posts: 38
Joined: 1 year ago
Location: East Yorkshire

Contribution Stars

Re: Brake servo locking up brakes!

Post by jcrcleeman »

Sorry for the confusion gents, I'm only just getting my head around the way servos work and their pressure values.
This is the one I fitted.


The discs are new so grabbing the rough edge isn't a factor custard and shims correct way up.

So, the ones at the site you mention should be OK, anyone any experience of them?
Attachments
Screenshot_2021-09-23-20-17-42.png
Screenshot_2021-09-23-20-17-42.png (88.08 KiB) Viewed 100 times
Screenshot_2021-09-23-20-17-42.png



User avatar
Sleeper
.
MMO Button
Posts: 549
Joined: 5 years ago
Location: Manchester U.K.

Re: Brake servo locking up brakes!

Post by Sleeper »

Could be the air valve ?

or lack of return spring..

John ;-)

P.S. Lockheed are good , pattern parts not so....



User avatar
jcrcleeman
.
United Kingdom
Posts: 38
Joined: 1 year ago
Location: East Yorkshire

Contribution Stars

Re: Brake servo locking up brakes!

Post by jcrcleeman »

Sleeper, do you mean the actual air valve line from the carb manifold or something else?

I've fitted a 1 way valve (the correct way!) In that.

Which return spring do you mean, the one in the chassis channel?



User avatar
Sleeper
.
MMO Button
Posts: 549
Joined: 5 years ago
Location: Manchester U.K.

Re: Brake servo locking up brakes!

Post by Sleeper »

I'll try and dig out my old notes from years ago about the Lockheed servos locking on.

John ;-)



User avatar
Custard
.
Ireland
Posts: 2952
Joined: 10 years ago
Location: Blackpool

Contribution Stars

The Cliff Lloyd Award

Re: Brake servo locking up brakes!

Post by Custard »

That one should be ok it is as Sleeper suggested 1.9 to 1 Ratio.

My first Morris was actual fitted with a second hand Lockheed one from a Hillman Hunter they are normally very reliable.

I am not sure why they should grab though.

I can understand if the valves don't release why they would stick on.

I would be tempted to try and find a nice hill on a deserted road, and with the engine off pump the pedal a couple of times to get rid of the vacuum and then roll down the hill and stop just to see if there is any grab then. If there isn't and the servo is new/just been fitted I would contact the seller to see if it could be exchanged.

I have borrowed a picture from the car builders website under their compact servo.

The air valve is in the white casing. Also if the return spring in the large air canister were to fail that would cause an issue too.

0025099_compact-single-circuit-remote-brake-servo.jpeg
0025099_compact-single-circuit-remote-brake-servo.jpeg (76.83 KiB) Viewed 91 times
0025099_compact-single-circuit-remote-brake-servo.jpeg


Normally when not braking the engine sucks out of the canister, shown by the blue.

But it is replenished by fresh air shown by the yellow into the valve, but what isn't shown is it flowing through the valve through the tube into the area where the blue arrows are.

So the diaphragm doesn't move as the pressure either side of it is equalised.

As soon as you apply the brakes the fluid passes up the purple line and closes off the air valve.

As soon as this happens the fresh air is still on one side of the diaphragm in yellow.

The engine sucks out the air the other side in blue.

So now the diaphragm moves adding pressure to the slave piston. But there is some resistance by the large return spring. These are quite strong around 2" diameter and probably double the depth of the canister from memory. I remember it being fiddly to compress and reassemble the canister on my own. So if that has broken it could produce your symptoms.

If the engine isn't running so there is no vacuum the diaphragm won't move, but there is a hole in the slave piston so fluid can pass through it allowing you to have unassisted brakes. When assistance is there the rod blocks that hole otherwise the piston would move forward, but the fluid could pass back through it and you would not get any assistance.

Hope that makes it a little easier to follow.

As far as I am aware the one way valve in the vacuum line is purely a safety measure to prevent any blow back from the engine pressurising the vacuum side of the diaphragm.

Sean.



User avatar
Sleeper
.
MMO Button
Posts: 549
Joined: 5 years ago
Location: Manchester U.K.

Re: Brake servo locking up brakes!

Post by Sleeper »

Nice one Sean.

There are just two other points, a helper spring was fitted to some air valves , putting extra pressure on the air-valve piston .
Is the air valve in the correct orientation...should be below the centre line..
servo1.jpg
servo1.jpg (30.25 KiB) Viewed 79 times
servo1.jpg
servo2.JPG
servo2.JPG (64.62 KiB) Viewed 79 times
servo2.JPG
EXIF-Data show/hide
Original image recording time:
2006:05:31 16:39:44
Image taken on:
Wed May 31, 2006 3:39 pm
Focus length:
5.7 mm
Shutter speed:
1/100 Sec
F-number:
f/2.8
ISO speed rating:
100
Flash:
Flash did not fire, compulsory flash mode
Camera-manufacturer:
FUJIFILM
Camera-model:
FinePix A330
Exposure program:
Manual
Exposure bias:
0 EV
Metering mode:
Pattern
John ;-)



User avatar
jcrcleeman
.
United Kingdom
Posts: 38
Joined: 1 year ago
Location: East Yorkshire

Contribution Stars

Re: Brake servo locking up brakes!

Post by jcrcleeman »

Thank you for that comprehensive explanation Sean. I at last have a grasp of how it all function's....or rather should function!

Now for some double checking of fitment and use.
The hardest part might be finding a hill....I live in the Holderness (flat coastal area) of East Yorkshire ! :D
John.



User avatar
jcrcleeman
.
United Kingdom
Posts: 38
Joined: 1 year ago
Location: East Yorkshire

Contribution Stars

Re: Brake servo locking up brakes!

Post by jcrcleeman »

Thank you John.
Yes I fiddled about for ages to make sure the angle was as you suggest.
Evidence of a helper valve would only be found by stripping it all down yes?
Test and recheck time now.
John



User avatar
Sleeper
.
MMO Button
Posts: 549
Joined: 5 years ago
Location: Manchester U.K.

Re: Brake servo locking up brakes!

Post by Sleeper »

John

Just five self tappers on the valve cover...

John ;-)



Post Reply