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January 31, 2017 at 2:30 PM

Midget Clutch Bleeding by John Ross                                  

jshross(at)binternet.com

Apart from the usual grouse of: “Midgets are made by midgets, to be used and maintained by midgets”, there is one particular bête noir of which most owners are aware:  the bleeding of the clutch system. It is a PAIN. It seems the design is aimed at making life very difficult for us: the main pipe run is mostly vertical and of more than adequate size (1/4 inch piping, instead of the usual 3/16 inch used for the brake piping); the bleed screw at the slave cylinder has one 2 mm hole through which fluid/air has to be bled, the so-called access plug in the driver’s footwell is not in the least bit accessible. One needs to be a contortionist to depress the clutch pedal whilst first finding and then releasing the bleed nipple through the access hole !

Putting some figures together, we find that the internal area of the pipe is almost 6 times greater than the area at the bleed screw: so the bleed screw on the slave cylinder is the main restriction. The difficulty is to get a fast enough flow for long enough to get all the air down the pipe and out of the (tiny) bleed screw.

After a frustrating week, with too much time spent underneath my Midget, I decided to revise the hydraulic system, whilst aiming to retain authenticity as far as possible. The plan was to ensure that ALL pipework from both the master cylinder and from the slave cylinder slopes upwards to a second bleed nipple located quite close to the master cylinder. While we can rely on death and taxes as inevitable, we can also rely on gravity as being quite a permanent feature. 

By this means, we can fill the entire system via the master cylinder reservoir and possibly the second bleed nipple; then wait for gravity to bring all the bubbles up to this point.

While you may want to re-work the existing clutch pipe, this is not really possible as the new pipe needs to be 30 to 40 mm longer.  It is also worth noting that in making the flared ends, the fittings can vary so they need examining to find if single flares or double flares are needed.  Single flares mate with concave seats, whereas double flares mate with convex seats.  The new items which are needed are listed at the end of this note. As usual, keeping everything very clean is essential.

 

Photograph 1 shows the original layout.

 

Photograph 2 shows the new layout.

 

 

 

What’s to be done ?

The details of the changes and the sequence of work are:     

1)   Remove the two bolts (9/16 in A/F) holding the slave cylinder to the clutch housing and

       decouple the push rod. This allows easy access to the existing bleed screw and also

       allows the cylinder to hang vertically. Push the operating rod into the cylinder and

       secure it using wire or strong cord, to keep the piston in the cylinder. It is best to remove

       the rubber boot from the slave cylinder but in this case you must have a mechanical

       stop to retain the piston in the cylinder (See “Comments B” below).

2)   Open the bleed screw and drain off the fluid. Close the bleed screw.

3)   Move the heater air duct out of the way and remove the ignition coil and, optionally, the

       distributor cap and the heater fan unit; it helps to move the starter motor lead so as to

       allow better access to the bottom end of the clutch pipe. 

4)   Remove the clutch pipe (and retain).

5)   At the top end of the clutch pipe, cut it at Point A (see photograph 1); fit a male union to

       the short section of pipe and produce the flared end. Gently twist this part of the pipe so

       that when it is refitted to the master cylinder the pipe will slope upwards over its full

       length and when the leg of the tee is fitted to this pipe, the mid-point of the tee is

       5 to 10 mm below the brim of the reservoir. Fit the pipe loosely to the master cylinder.

6)   Before using the tee, it is suggested that it is drilled out to 3.5 mm diameter.  Fit the tee

       piece loosely to the other end of this section of pipe, using the leg of the tee. 

       The arms of the tee will be tilted slightly towards the rear of the car.

7)   The new clutch pipe comes with the unions attached and the flares already made.

       Using the old pipe as a guide, bend the new pipe to suit the fittings to which it will mate,

       starting at the bottom end. The top end of the pipe will connect eventually to the

       downward facing arm of the tee. When you are satisfied, cut the top end of the pipe to

       length, fit the union and produce the flare to mate with the tee piece. This pipe should

       be rising at all points from the slave cylinder to the tee.  Fit this pipe and secure it with

       the pipe clip.

8)   Fully tighten all union fittings, keeping the tee in the correct position (see 5) above).

9)   Taking the bleed screw, it seems worthwhile to enlarge the bleed hole to 3.5 mm and

       drill it through to give two bleed holes.  Using plumbers PTFE tape, wrap the top of the

       screw with 8 to 10 turns, fit it to the upper arm of the tee piece but don’t close it fully. 

       Fit a 1.5 m length of 5 mm dia clear plastic pipe to the bleed screw and fix it vertically.

10) Fill the reservoir on the master cylinder with fluid, after decanting it first into a clear

       container and waiting for any bubbles to disperse.  Silicone fluid has a very

       worthwhile benefit as it does not absorb water and does not damage your paintwork. 

        It is not recommended to mix hygroscopic (e.g glycol) with non- hygroscopic (e.g. silicone)

        fluids in the system.  However, any traces of glycol fluid in the components would not

        cause an adverse reaction with the silicone as they are both DOT rated fluids.

Bleeding sequence

11)  Loosen the clutch reservoir cap. Using a small vacuum pump, draw about 20 to 30 cm

        of fluid up the bleed pipe and wait  for any bubbles to disperse, keeping the reservoir full.

        Repeat this until no more bubbles appear.

12)  With fluid in the bleed pipe, depress the clutch part way and keep it there. This closes

        off the reservoir from the copper tube feeding the tee piece. Push the slave cylinder

        piston (SCP) fully into the slave cylinder and hold it there; this pushes fluid and air up

        the bleed pipe.  Wait for the bubbles to disperse then release the SCP which slowly

        falls under its own weight to the bottom end of the cylinder.

13)  Operate the clutch normally, this is to expel any bubbles from the short pipe feeding the

         tee piece; wait for bubbles to disperse then bring the clutch up.

14)  Repeat 12) and 13), keeping a column of fluid in the bleed pipe, until the system is free

        of bubbles.

15)  Close the top bleed screw.

16)  Re-fit the slave cylinder to the clutch housing and couple up the push rod.

17)  Operate the clutch pedal normally several times.  The travel of the clutch operating arm

        should be at least 8 mm, measured at the clevis pin. This is important as there is very

        little excess travel allowed. 

18)  Check / top up the master cylinder. If the fluid level in the reservoir is too high, slacken

        the bleed screw and remove some fluid via the top bleed pipe by operating the clutch

        pedal; close the bleed screw and remove the pipe.

19)  Refit the other parts which had been removed.

20)  In future, checking for any air in the system can be done very simply by fitting the bleed

        pipe, releasing the bleed screw and drawing some fluid and any air up the bleed pipe.

Items which will be needed to convert from standard set-up:

Item                 Details                     Supplier        Supplier’s part number                    Comments

1	Clutch pipe ¼ inch x 20 g inc. 2 male unions	Moss	AHH 6362 C	1 off
2	Tee piece, 7/16 UNF female	Automec	HU34 Female tee piece 7/16  x 20 UNF with concave seat	1 off,   Enlarge drillings to 3.5 mm, optional
3	Male pipe union, 7/16 UNF male	Automec	HU14 Male 7/16 x 20 UNF to suit ¼ in pipe	1 off
4	Bleed screw, 7/16 UNF	Automec	HBS004 Bleed screw 7/16 x 20 UNF to suit tee piece	1 off,  Enlarge bleed hole to 3.5 mm, optional.
5	Silicone clutch/brake fluid, 1 litre, DOT 5	Automec or Moss	Automec: Silicone DOT 5         brake and clutch fluid                         Moss: ABF4	As required
6	Pipe Flaring tool			Optional :  most garages can do this for you if need be
7	PTFE tape	D I Y
8	Clear plastic bleed pipe	D I Y		1.5 m x 5 mm dia.

Comments

A)  Some figures may be useful.  The amount of fluid displaced by the master cylinder

      = 4.6 cm ³  (a 235 mm length of the 5 mm dia bleed pipe).  The slave cylinder I.D. is

      25.4 mm. so the maximum theoretical travel of the piston is limited to 9.1 mm. 

      This is pretty marginal and goes to explain why bleeding of the clutch etc. is the pain it is. 

       If only they had used 22 mm !  That would have given a maximum travel of 12 mm and

       allowed a 2 mm clutch return limit stop clearance to be used.

B)  When I did a complete refurbishment in 2006, I fitted a (removable) mechanical stop to

      the slave cylinder to prevent the piston from falling out accidentally. This took the form of

      a couple of 2 mm dia pins which were across the cylinder where there is a groove for

      the rubber boot.  This worked fine but during the current work, I found that these pins were

      on verge of limiting the travel of the piston.  Bad news !  But  then I found that the possible

      travel of the piston in the slave cylinder was around 36 mm.  Massive, compared to the

      usual operating travel of 8 mm.  So the best remedy was to keep the pins but extend the

      slave cylinder operating rod by about 6 to 8 mm.  Note: this did not increase the

      operating travel, it just moved the piston further into the cylinder.

C)  In 2006, I fitted a clutch return limit stop (as also seen in the original Mini) to try and

      ensure that all the  travel could be used to operate the clutch. 

      This seemed worthwhile.

Acknowledgements:  The photographs were taken by Colin Grant, MGCC Deputy Editor, who also arranged the text layout and made helpful comments.

Automec very kindly supplied items 2, 3, 4 and 5 in the above parts list at no cost.

Contact details:

Automec: Automec Equipment & Parts Ltd, 36 Ballmoor, Buckingham Industrial Park, Buckingham, Bucks, MK 18 1RQ, United Kingdom. www.auotomec.co.uk Tel +44 (0) 1280 822818

Moss Europe: Various branches around the United Kingdom. www.moss-europe.co.uk/

Tel +44 (0) 1179 232523

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