Remote brake servo - improving it's action

[vitesseefi]

When I rebuilt my GT6 Mk3 a couple of years ago I decided to add a remote servo. At the same time I upped the master cylinder size from the factory 0.7" to 0.75" with the aim of retaining a nice short, firm pedal feel. The rest of the braking components are new. The pads are standard Mintex.

The brakes work very well, with one caveat. The pedal feel is indeed firm and short travel, but after some moderate initial bite, with moderate pedal pressure, it's necessary to apply a quite alot more pressure to increase the effect - whereupon the servo suddenly wakes up and you get more than you were aiming for - it's not a violent effect, but it's impossible to properly modulate. If you want to stop in a hurry, no problem, you can stand it on it's nose

What I'm not sure about is whether it's a problem I've created with my choice of components, or whether there is a problem with the servo itself. I'm inclined to think the latter, and if this is the case, I wonder if the servo can be fettled to improve it?

This is (probably) the offending item






and the car itself



Any ideas on how to sort this issue gratefully received

Thanks

Nick

[comfortablynumb]

How were the brakes before you fitted the servo (assuming you managed to drive it before restoration)?
What happens if you take the servo out of circuit?
Since you changed two things at the same time I'd be reverting back to standard and see how it behaves then?
Just my twopenneth

[vitesseefi]

Thanks for your reply. The car came to me as a heap of rusty and bent bits so I’ve only driven it in its current form.

The problem seems to be that the servo comes in later and harder than it should but I’m unclear whether it’s a fault or feature. It releases fine.

I run the same basic system on my Vitesse with no servo. Until I discovered M1144 pads, this was a “both feet and prayer” set-up. With the 1144 and an undersize 0.625” master they work pretty well albeit with a longish pedal and needing a firm shove.

I’m pretty sure if I remove the servo from the GT6 it will become a “both feet and pray” system also……

Nick

[comfortablynumb]

Can't see from the pic, but is the servo the type you can dismantle?
Strip it down and have a look?
When I fitted a remote servo to my MK2 Cortina I've got a feeling that I had the same sort of results, but that was a standard master, and 30+ years ago!
On one servo I looked at a couple of years ago there was a small gauze filter for the vacuum, could that be restricting the vacuum until it's overcome?

[vitesseefi]

Believe it’s a Lockheed style one, though perhaps not actually made by Lockheed these days. It’s the general purpose one available from various sources. 1.9:1 assistance ratio iirc. Prepared to take it apart but would prefer to have a seal kit to hand, which might be a challenge…….

Conventional DOT4 fluid in the system.

Nick

[biturbo228]

The only thing I could think of that might cause those sorts of symptoms is if the piston is just a little sticky in the bore, a little bit down from where it sits at rest.

So you press the pedal, it moves a bit and the brakes engage, but the piston gets a bit sticky. Press harder and eventually the servo pushes it past the sticky patch and you get full brakes.

But you'd expect it to be sticky in both directions so maybe it's not the bore but something catching in the servo itself...

Not sure! Wonder what the inside of one looks like...

[v8ian]

I have built a few brake systems over the years, and personally wont fit a servo. OK, the servo decreases the pedal pressure and makes it easier to push, but it also decreases the feel, I will play with pedal ratio and cylinder size till I get what I like.

[comfortablynumb]

Oct 12, 2021 8:42:44 GMT vitesseefi said:

Believe it’s a Lockheed style one, though perhaps not actually made by Lockheed these days. It’s the general purpose one available from various sources. 1.9:1 assistance ratio iirc. Prepared to take it apart but would prefer to have a seal kit to hand, which might be a challenge…….

Conventional DOT4 fluid in the system.

Nick

I did have servo kit kicking around here, bought in error and never used, sods law means either I can't find it or its the wrong one, but I'll have a look when I get chance

[ferny]

If it's the cheap replacement (power boost or something?) They come apart easy. I had to strip mine down and clean it when it filled up with oil...

[vitesseefi]

I did originally buy this servo for the Vitesse, but couldn’t find anywhere to mount it that wasn’t horribly obstructive or looked pants. I mostly got around the problem in other ways, but not ideal and my wife still moans about the pedal effort needed when she drives the car.

Hence using the servo this time around, hoping for a more wife-friendly outcome. My first encounter with a remote servo, and not liking it much so far, though to be fair, if this step in the action can be eliminated or even much reduced, it would be fine.

Might end up binning it yet, but I find it hard to believe they are all like this.

Nick

[vitesseefi]

Oct 12, 2021 9:42:25 GMT ferny said:

If it's the cheap replacement (power boost or something?) They come apart easy. I had to strip mine down and clean it when it filled up with oil...

Yes, probably the same one….. does yours operate progressively Pete or is there a step in the action?

Nick

[gtviva]

They are available in different ratios as well so maybe you need a different ratio, cant quite see how its mounted but the other thing that helps is to keep the nose up to get the air out when bleeding

[vitesseefi]

Nose is up slightly. In fact, having come upon the proper Lockheed fitting guide for it online last night, I seem to have (by chance mainly) complied with their installation recommendations.

It’s bled fine with a nice short, firm pedal. 1.9 is the ratio Lockheed recommends for all small chassis Triumphs and most other things besides.

Nick

[colnerov]

Hi, Ahh yes, now you mention mounting requirements I can remember someone elsewhere on another forum asking about which way up to mount it in regards to the air valve being at the top or the bottom. After doing some research for my own interest I can can say 'it depends'. Apparently when you depress the brake pedal the pressure in the hydraulic side of the servo pushes a plunger which operates the air valve to allow atmospheric air pressure to assist braking effort. However The 'O' ring on the plunger can let some brake fluid through into the vacuum chamber, so a mod by the manufacturers was to fit two 'O' rings. However if the first one let fluid past the pressure between them can restrict movement of the plunger and hold the brakes on.

Now all that was a while ago so is from memory. Broadly speaking it's advisable and common practice to have the air valve uppermost. But do your own research to make sure.

Colin

Edit: I seem to remember it was explained on a Lotus forum, sorry I can't be any more help than that.

[vitesseefi]

Limora fitting instructions here. Seem to be identical to the Lockheed ones I found last night apart from the Logo

www.scparts.co.uk/pdf/533613.pdf

Nick

[ferny]

Oct 12, 2021 10:05:02 GMT vitesseefi said:

Oct 12, 2021 9:42:25 GMT ferny said:

If it's the cheap replacement (power boost or something?) They come apart easy. I had to strip mine down and clean it when it filled up with oil...

Yes, probably the same one….. does yours operate progressively Pete or is there a step in the action?

Nick

I'd like to provide a useful answer, I really would, but... you know my driving style! 🤣

I find that I can feel the brakes come on and slowly improve as they warm but with no input of foot movement. I felt it with the 1144 and more so with 1155. My co-driver this year also noticed.

I drive the Acclaim and a '05 Peugeot Expert regularly as well, both obviously have a servo. I can't feel any noticeable difference between any. Certainly not a sudden effort from the system as you describe.

Have you tried without the vacuum connected? It could be an iffy non-return valve. My next step would be a temporary brake line bypassing the servo. I'm sure you'll have the odds and sods to knock something up easily. Even if only for your own amusement.


Also. My instructions look the same as yours just mine are less polished. Mine was bought off eBay (£90 or such?) And came in a blue box. It was advertised as fitting Triumphs, MG's, Space Shuttles, etc. Same ratio as yours too.

[ferny]

Oct 12, 2021 14:28:48 GMT colnerov said:

Hi, Ahh yes, now you mention mounting requirements I can remember someone elsewhere on another forum asking about which way up to mount it in regards to the air valve being at the top or the bottom. After doing some research for my own interest I can can say 'it depends'. Apparently when you depress the brake pedal the pressure in the hydraulic side of the servo pushes a plunger which operates the air valve to allow atmospheric air pressure to assist braking effort. However The 'O' ring on the plunger can let some brake fluid through into the vacuum chamber, so a mod by the manufacturers was to fit two 'O' rings. However if the first one let fluid past the pressure between them can restrict movement of the plunger and hold the brakes on.

Now all that was a while ago so is from memory. Broadly speaking it's advisable and common practice to have the air valve uppermost. But do your own research to make sure.

Colin

Edit: I seem to remember it was explained on a Lotus forum, sorry I can't be any more help than that.

This is interesting.

At times I used to feel the brakes hold on briefly after using them. It doesn't do it now and it possibly stopped after stripping mine down.

Thanks for the info.

[misterm]

If it is one of the cheap Chinese non-genuine ones from eBay and the like ( I think I paid about £57), I have seen it mentioned on a make specific forum (can't remember which one) that the piston in the cylinder can be sticky in operation and it is worth popping the cylinder off the vacuum tank and feeling for smooth travel before fitting.