brake master cyl upgrade

[triple tango]

With an old Dolly I've just aquired came a supply of old Dolly Mixtures. In issue 104 (Sept '04) page 45 is an article on fitting a Ford P100 master cylinder. It appears to be a bolt on swap, has a larger bore of 22.2mm (Dolly 19mm) giving 36% more fluid movement while retaining the Sprints 38/62 split.
Seems a good idea. I run a "Trackerjack" kit & do find the pedal travel is more than a standard Dolly. Perhaps a P100 master cylinder will cure this.
Comments please before I go & buy one.

[Purplebargeken]

I'm presuming this would be a Sprint only upgrade and wouldn't be applicable to both dual brake line and single? Just a thought as it would be a good mod overall.

[Jod Clark]

I believe the fluid unions are a different thread. Its only a replacement for th dual line cylinder as well.

[tinweevil]

I'm sure I'm stating the obvious for you Roy but for everyones benefit.....

You'll increase the pressure required on the pedal to achieve a given rate of retardation.

Say for the sake of explanation you apply a force of 10 lbs to the pedal and the pedal provides a mechanical advantage of 10:1. This means you apply a force of 100lbs to the brake master cylinder. I'll ignore the servo because a) it's a constant in this and b) unlike modern car designers we quite like our cars to be stoppable in the even of servo failure. So 100lbs is applied to the brake master which has a cross sectional area of 0.44 sq in so the pressure you create in the pipes is 227.55 lbs/sq in. With the larger ford cylinder the same pedal effort results in a pipe pressure of 166.68 lbs/sq in. Since the pipe pressure is lower you get lower clamping pressure at the calipers for your 10lbs leg effort.

So yes, you will have to move the pedal less distance to achieve working contact at all 4 corners but you will have to push harder to stop the car. No surprises the increase in effort is in the same ratio as the bores, 36%, so 13.6 lbs of leg effort achieves the original stopping power.

I would be inclined to look at the bore of the standard vs ford brake calipers and pad areas to see what you have already done there. It could be that changing the master will move the overall mechanical advantage of the system (pedal to pad) closer to standard in which case IMO this would be a good change. OTOH if you are moving the overall values further from standard it could be, especially in the event of an insurance claim, a bad thing.

It's worth considering the caliper to rear cylinder bore ratio as well. The trackerjack mod will have changed the work ratio of front to rear brakes, changed that 38/62 split. Fine if the rears are working relatively a little less than standard as the fronts are certainly up to the greater work share. Less good if the reverse has happened. You can adjust this by changing the bore of the rear cylinders, I think other bores are available to fit?

Any use?

[tinweevil]

Fine if the rears are working relatively a little less

That's not meant as criticism of the TJ mod or anyone who uses it. The TJ mod is a track proven mod that most certainly improves the brakes of a Dolomite. IMO.

[triple tango]

I think its standard practice to remove the rear brake valve when doing a "Trackerjack" mod anyway.

[jeroensprint]

Changing the rear cilinders is solving the problem. Sprint has 19mm cilinders and 1850 17mm. Early 1850 had 15mm and that are the ones you must use. When removing the rear brake valve the rear will brake ok. I have some dolomites with double calipers on the front and had the same problem and solved it this way.

Jeroen Rothman

[sprint95m]

I agree with Jeroen.
This is based on my experience of a Trackerjack conversion on my early 1850 retaining the smaller bore 1850 wheel cylinders. The brake pedal on my car does not have excessive travel. However being an early car, it has a single line master cylinder, so if you have to use a dual line cylinder my observation is probably irrelevant.

I fitted Jaguar XJ12 calipers to a T2500 a few years ago and that one definitely needed a bigger bore master cylinder to compensate for the increased caliper pistons' working area. On that car I used a Sherpa servo and 7/8" bore dual circuit master cylinder. The pedal effort felt the same to me but more importantly the stopping distance was dramatically reduced.

[Sprint36]

Got a P100 master cylinder on the Orange car. I belive the unions are 10mm, but I didn't fit it so can't confirm. The connections come out the other (left hand) side.

David

[merlind100]

Very intersted in this thread as I have both TJ brakes and a master cyl issue to deal with. My car by the way runs without the rear bias valve.

[merlind100]

Decided to try this but have as yet not been able to find one. It may now be an obsolete part

[jeroensprint]

Hello, some Jaguar xj series 1 or 2 have a dual master what fits on a dolomite But have a larger bore. Dont know exacly wat type of xj.

Jeroen Rothman

[merlind100]

I have been absolutaly unable to find one of these cylinders new. Finally found one in a breakers yesterday and thought great I'll get that and reseal it then the guy said he wanted £45. It's still there and I'm now looking at other alternatives, if I come up with anything I'll post it

[RichardHyde]

Just bled my Trackerjack brakes and have been researching the slightly longer pedal travel.

Does anybody know the part number for early 1850 15mm rear cylinders ? Do they make much difference compared with standard Sprint 19mm cylinders.

Drilling a new hole for the clevis pin 15mm lower than standard was another idea.

Thanks, Richard

[xvivalve]

Whilst it is only a difference of 4 mm, it represents a reduction in XSA to 62%, so the travel of the wheel cylinder piston will be increased by a proportional amount thereby reducing pedal travel before the shoes lock with the drum.

Up to WF68200 used GWC1502 (same as 1300 and 1500), after that VIN the 1850 used GWC1121 (same as Sprint)