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Master cylinder diameter for big brakes

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Westfield_XI

Westfield_XI

Jedi Warrior
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I am running 4piston Toyota Fourunner brakes with MGB GT rear cylinders on my Spridget suspended Lotus Replica. Yesterday I had a master cylinder failure and thought I would take advantage of the opportunity to make an upgrade. I have single circuit brakes with the same size MC as the clutch. I sourced them from a fellow named Colin Dodds in Australia and I have forgotten the diameter. I do know that they were a direct replacement for the stock 1967 part.

The stock MC in conjunction with the big brakes was never very good needing long travel to get much results, this I assume was due to the larger piston area of the bigger discs. I would suspect that I am similar to the piston area of a set of Willwoods and would need a similar size MC. Do any of the racers or modifiers have any suggestions of what size MC I should order? I plan on getting a standard size Tilton as I believe it will be an easier install onto the Spridget pedal box and they are available in many diameters.

I have a call into Wilwood and also to Tilton tech support, but just in case, I wanted to hear from the forum if they have any tips.
 
Westfield_XI said:
...

The stock MC in conjunction with the big brakes was never very good needing long travel to get much results, this I assume was due to the larger piston area of the bigger discs...
Click to expand...

Your assumption is correct. You would need a master with a larger bore. However, you don't get something for nothing. With the pedal travel decreased because of the larger bore, the force necessary to apply the brakes will go up.
 
Thanks, but my real question was what diameter should I get..... I am aware of the need to move more fluid for the larger piston area vs the greater pressure required to move that fluid. In my case, the car is very light and stops easily, I just would like the pedal to not feel like there is a small rodent under my foot when I brake. It feels "squishy" as it is now.....
 
It's normally 3/4 on disk brake set-up , where as drums had 7/8. Also checks Gerard's site
 
With a four-pot caliper on the front and most likely 7/8" ID wheel cylinders, the size I would select would be at a minimum a 7/8" diameter. Cylinder diameters are also available in 1.0" and 1.125", although the supplier list starts shrinking. Also be aware that these after market cylinders typically come in straight up and down mounting while stock type Lockheed cylinders in a 1967 Spridget would have been at an angle.

Might be worth considering a brake bias valve for the rear to adjust braking effectiveness at the rear.

I would hesitate to think that "Squishyness" is cylinder size related. This is usually air intrusion related. This may be the cylinder bottoming out on the rubber sealing cup.

HTH,
Mike Miller
 
Thank you, Mike,

I called Tilton and they said that mounting the MC at an angle is no issue as long as the ports are submerged in brake fluid. I plan on using the Tilton 74 series universal MC since it comes with large and small reservoirs as well as a remote reservoir for around $80 or so. I also spoke with Rivergate and they suggested that I go to a larger clutch cylinder to decrease the pedal travel issue I have. Might as well do both at once!
 
I've been very happy with my Tilton 3/4" brake and clutch master set-up. They are of good quality and relatively cheap and mounting is easy...very good pedal feel as well.
Rut
 
Rut, what brakes do you have? Did you have to do both cylinders at once? I am worried that the tilted reservoir will interfere with the upright one if I just change the brake one.
 
I have the old style rear drums now, but will upgrade to the later dual style with a residual valve. The Tilton 74 series lends itself to a variety of mounting positions and is very easy to work on. I've even considered going with a Tilton hanging pedal assembly with the bias valve since I have 3 of the 74 MCs. I don't know that I will ever need to fine tune my rear brakes that much, but it's an option.
Rut
 
You can get just about any size bore on the Tilton 74 series MCs so you can mix and match.
Rut
 
nomad said:
Stock BE is 7/8 on both bores!!
Click to expand...

What is stock for a '67 Midget? That will give me a starting point.....
 
Westfield_XI said:
nomad said:
Stock BE is 7/8 on both bores!!
Click to expand...

What is stock for a '67 Midget? That will give me a starting point.....
Click to expand...

Don't have one but imagine it is 3/4 for discs as on the earlier cars.

Kurt.
 
I would agree that M/C size for disc brake equiped cars with the formed metal pan for separate brake and clutch master cylinders is most likely 3/4". I can't remember the cylinder tail shape coding for that specific Lockheed cylinder. But, I looked on one of the internet parts vendors database and it supports a single 3/4" diameter cylinder as being used prior to 1968. After that, it was a dual circuit 3/4" cylinder.

A web search should produce an image that shows how the different Lockheed cylinders are shaped at the ends where the fluid outlet is located.

My recommendation on upsizing to the 7/8" cylinder is still a reasonable next step. A 1.0" cylinder would likely reduce the pedal stroke even further.

HTH,
Mike Miller
 
Mike, I am leaning to the one Inch one for just that reason, but have a small concern about higher pedal effort. But then again, how high could it be on a 1200 pound car with a pair of 4 piston discs up front. I will just have bite the bullet and give it a try.
 
I went with the 1" M/C from Tilton, it was a bolt on install except for the actuating rod. The Tilton comes with a long threaded rod and the Sprite pedal box needs a clevis. I cut the Tilton one down to the correct length, cut the clevis off of the Sprite rod and had my local welder put them together. The Tilton reservoir is cocked about 25 degrees or so towards the passenger side of the car where the Sprite one was bolt upright. I do not have room under my hood to use the remote reservoir, not enough height to ensure flow to the M/C, so I used the smaller of the two on cylinder reservoirs supplied. I have enough fluid above the inlet ports to keep them submerged so I think that will work just fine.

I have MGB GT rear brake cylinders in conjunction with Toyota 4runner front calipers and the pedal effort has increased noticeably. Not too much mind you but you can feel the difference...... I spent 65 miles in the canyons this morning and had no problems braking, but it sure would take a firm push to lock up the fronts! The stock piston was 3/4" so I went up by 30% which is a lot. I am tempted to put this one onto the clutch and try a 7/8" M/C on the brakes to see how it feels. The brake feel is much improved though, no squishy feeling.....
 
Westfield_XI said:
I am running 4piston Toyota Fourunner brakes
Click to expand...

13WB's? Man, that's a thick caliper, you running spacers on the wheels to clear it? I was trying to put those on the Jav but needed too much spacer.
 
I do not know what a 13wb is, but all I had to do was grind off a protruding lug on the piston housing to make them fit behind my 15" wires. They are spaced off center towards the onboard to help. They are 1985 IIRC. Very cheap at Autozone, I have not had to look for upgraded pads yet so I do not know what is available. I am sure I will be able to find something though when these are ready for replacement. So far they are more than up to stopping my 1250 lb car.
 
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