Clutch pedal servo

[Bill MullParticipant @billmull]

Hi All,

I thought this might be a good place to ask about a query that came up in discussion with a colleague,

He has a tractor with a hydraulically operated clutch, a traditional style master cylinder to slave cylinder on the bellhousing arrangement . The issue is that the clutch pedal is very heavy and always has been, it seems to be the way with this model tractor.

This got me thinking , would a brake servo device help to reduce the pedal effort required? I remember remote brake servo kits being advertised in 1970s car magazines ,the Girling Powerstop being one unit I remember.

Does anyone have first hand experience of such devices and is there any reason why a remote add on brake servo unit wouldn't work in a hydraulic clutch application?

[Bill MullParticipant @billmull]

[john halfpennyParticipant @johnhalfpenny52803]

Yes, you can add a vacuum servo, but if the tractor is a diesel you will also need a vacuum pump – maybe electric? Girling servos are not available, but replica Lockheed remote servos are sold new in a range of sizes – probably the smallest will do the job.

[bernard towersParticipant @bernardtowers37738]

yes John you are right, in the 70s our vacuum brakes with Diesel engines had an exhauster fitted the opposite side Ito the alternator to effect a vacuum. Fitted a remote Girling Powerstop to my Mk 1 GT cortina then it would outbrake anything!!

[john halfpennyParticipant @johnhalfpenny52803]

Casting my mind back (I was a Lockheed apprentice), the old Manumatic system used a gearlever actuated vacuum servo to disengage/engage the clutch, thus dispensing with a clutch pedal. The system used pivoting bob weights to engage the clutch from rest, and this was the achilles heel because it was possible to drive away in top gear (with massive clutch slip and consequent wear). The system worked well if used correctly, but the gearchange was rather slow.

[lee websterParticipant @leewebster72680]

Some years ago a friend had electric power steering fitted to his Mk2 Jaguar. Perhaps something similar is available for power assistance to brake and clutch pedals?

[Paul LousickParticipant @paullousick59116]

Is it possible to fit a smaller diameter master cylinder or bigger dia slave cylinder to get a better mechanical advantage. It will however require a longer pedal stroke to pump the same volume of fluid. Or adjusted so there is no slop and lost travel in the system.

[HopperParticipant @hopper]

+1 on what Paul Lousick says above. I have found that reducing a motorcycle mastercylinder diameter by as little as 1mm can make a noticeable difference in brake performance, requiring much less lever pressure. 2mm even more so.

Or is it possible to reposition the master cylinder or pedal pivot so the distance from pedal pivot to master cylinder is reduced, thus giving more leverage and making the clutch "easier"?

[martin haysomParticipant @martinhaysom48469]

Posted by Bill Mull on 28/07/2022 20:56:13:

Hi All,

This got me thinking , would a brake servo device help to reduce the pedal effort required? I remember remote brake servo kits being advertised in 1970s car magazines ,the Girling Powerstop being one unit I remember.

Does anyone have first hand experience of such devices and is there any reason why a remote add on brake servo unit wouldn't work in a hydraulic clutch application?

the tv program car sos did this on a mk1 zodiac

[Dave HalfordParticipant @davehalford22513]

The Mitsubishi GTO had a clutch servo

[Nick WheelerParticipant @nickwheeler]

Posted by Hopper on 29/07/2022 03:04:09:

+1 on what Paul Lousick says above. I have found that reducing a motorcycle mastercylinder diameter by as little as 1mm can make a noticeable difference in brake performance, requiring much less lever pressure. 2mm even more so.

Or is it possible to reposition the master cylinder or pedal pivot so the distance from pedal pivot to master cylinder is reduced, thus giving more leverage and making the clutch "easier"?

Another plus one from me too; basic master cylinders are pretty much interchangeable available in lots of different bore sizes and don't cost much. So a simple bolt-on part should do everything you want without the time and work of engineering some assistance.

Electric steering cloumns use a motor and gears directly onto the shaft, so I don't see how that could possibly help?

[john halfpennyParticipant @johnhalfpenny52803]

Just out of interest I priced up the parts. You should be able to fit a pump/servo for under £150 including a few hydraulic fittings and some vacuum hose. The advantage of this solution is that the parts can be fitted/hidden anywhere. I have played with bore ratios and the load/travel compromise, but typically the problem is finding a cylinder with the correct external mountings. You may also need metric and imperial fittings for opposite ends of the pipes.

[Jon LawesParticipant @jonlawes51698]

I assisted in converting an old motability car to a rally car a few years ago; it was largely unmodified from stock for the gentlemans disability other than an inline servo in the clutch line. It was a Mk2 escort.

[Chris Evans 6Participant @chrisevans6]

I modified a series three Land Rover diesel to run an electric vacuum pump. Triggered the pump from the brake light switch via a relay. Worked very well, pump was a Hella "U28" these are fitted to Volvos amongst other vehicles.

[Bill MullParticipant @billmull]

Thanks for the interesting response everyone, I had thought of changing the master cylinder diameter but the master cylinder is fitted in a pretty awful location under the dashboard on the cab bulkhead , access is terrible.

That's why the idea of a remote servo appeals to me as there is plenty space under the cab to mount the servo unit and access to the clutch slave cylinder is fairly straightforward .

I ve been reading up on this online today and it seems to be a common modification to older land rovers which appear to suffer from heavy clutch pedals , I have discovered that there are different servo " boost" ratios 1.65. 1.9 2.3 etc. , I would guess the more boost the better in this application

[Howard LewisParticipant @howardlewis46836]

A servo will require some thing to power it, compressed air, vacuum, hydraulics or electrics, bringing in extra components and complication.

Incorrectly set up, the system might start to "hunt" whilst being operated; if possible keep things as simple as possible

Changing the master cylinder to a smaller diameter, or fitting a larger diameter slave cylinder will mean that although the mechanical advantage is increased, more pedal travel will be needed. The same would happen if a longer operating lever were fitted to the lever on the clutch throwout shaft.

Can the extra clutch pedal travel be accommodated.? Could nthe clutch pedal be lengthened, instead?

The Bristol RE bus with dry plate clutch needed an air operated servo to reduce pedal loads, but heaven help you when the cylinder failed!

The Leyland Leopard clutch operating system only JUST provided the required travel, and needed to be set up very carefully…

Changing from hydraulic operation to purerely mechanical increased the travel,, at the expense of increased pedal load. But the increased clutch life (Better clearance, making for easier gear changing, and less incentive to ride the clutch ) made it a worthwhile modification.

Howard

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