Upgrading XK140MC Master Cylinder to Tandem

XK
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Finally started the brake master cylinder upgrade on my Jaguar XK140MC. Since I had already done the Wilwood disc brake conversion I wanted to change to a tandem master cylinder. I know there are folks that say this is not necessary but I see no down side to it.

First of all I want to say that on all of the upgrades I have done I make sure they are completely reversible since this is a numbers matching car. I always save all the original parts I remove and box them to save and keep available should the need ever arise.

My plan is to use a master close to the original size but also change the pedal ratio for better feel and pressure. I will fabricate a mounting bracket that utilizes the original space.

I took the brake pedal and drilled another pivot hole for the master cylinder pushrod. I moved this .750” toward the bottom. This will change the pedal ratio from 4.66 to 6.22. Most of the info I found on the web calls for a pedal ratio of 6 to 7 for manual brakes. I knurled a 3/8” rod and pressed it in the original pivot hole (removable) so as to not have a weakened pedal. Using a 1” bore master and changing the pedal ratio will give right at 800psi which is what is called for on disc brakes. Also, the reason for using a rear proportion valve to lower the pressure/volume to the rear drums.

I used a Wilwood 1” bore tandem master with vertical mounting holes and remote reservoirs. This will allow the master to align with the pedal pushrod. I will add a diagram of the bracket that I fabricated along with part numbers for purchased parts.

I am also installing an adjustable proportion valve for the original rear drum brakes. The one I used has a brake light switch included. Since the master cylinder reservoirs are mounted high in the original spot I am not using residual valves.

The mounting bracket uses the same place on the frame but I did drill and tap a new mounting spot 2-3/8” C to C from the front hole because the rear hole falls where the new master sits. You also have your move the master down to keep it centered with the new pivot on the pedal. I would advise that you make a pattern for the part of bracket that goes to pedal mount bolt out of cardboard. Then make that part and final fit and tack weld in place after building the main part of mounting bracket since there may be some minor differences on each car. Then remove and do final welding. Make sure not to weld bracket where master sits as it will interfere.

I used a piece of 1”x1-1/2” aluminum tube to mount the remote reservoirs. Used the original holes from the Jaguar reservoir and added two holes in the tube to match mounting on reservoirs.

I hope this helps anyone wanting to make this change. I still have to bleed and test but will have to wait for better weather conditions.

Parts used:
Wilwood, 1” bore tandem, remote reservoir master cylinder 260-7563

Inline Tube, proportion valve with brake switch ADJP-05

2 each, 3/16” male inverted flare to 1/4” female inverted flare adapters

Also need a 7/16-20 inverted flare nut since I am saving all the original brake lines. It will be about .005” smaller than 1/4” tubing so I had to ream it out and polish to fit tubing.

3/16” brake line and nuts. 3/8-24

1/4” brake line and nuts. 7/16-24

1 piece 1”x1-1/2”x 6” aluminum tube

3/16”x5”x18” hot roll plate steel

Assorted hardware

Dot 4 brake fluid

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2 Likes
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Great looking installation. I always appreciate detailed tube bending.

You note that you will not be running residual valves because the reservoirs are mounted high. I am assuming that is in relation to fluid siphon. Not in reference to a residual pressure valve.
I have always installed a residual pressure valve in the rear circuit to over come the drum’s return spring retraction. Without the residual pressure valve the rear requires more travel to get the shoes to the contact point while the discs are already engaged. Do you have something in the dual cylinder circuit to compensate for that?

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@Essextrp
I do have the residual valves 10# for rear and 2# for front. But I wanted to wait and see how the system acts before or if I need to install. I have found that if you maintain well adjusted rear brakes the system works well and the proportioning valve does the work of applying the correct pressure. Thanks for your comment.

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Dan, you have everything under control. I like your note about keeping the shoes adjusted which solves most of the requirement for residual line pressure.

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It looks like a very danger’s situation this way, when looking at the brake pipes. They stick out under the chassis.
This will be a direct failure for MOT

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@RuschP The angle of the photo makes it look that way. Here is another angle. I am also planning a skid plate attached to bottom of frame to extend out under the master. Here is another angle. Not my first rodeo.

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Very nice work Dan.

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Nicely done, I took a similar approach but used twin master cylinders 5/8” F 1” R with a balance bar and increased pedal ratio as you did. The smaller diameter front master cylinder nearly doubles the line pressure to the front discs with the same foot pedal pressure and the rear remains the same but is adjustable via the balance bar. Greatly reduced pedal effort with excellent modulation.

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100% reversible if someone would ever wish to.

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