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Old 08-18-2006, 06:46 PM
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Pedal Travel

Probably has been asked before, but:

Just completed the brakes on my '64 Mini van with bigger 4 piston calipers up front and Fiero calipers in back. Using the original Mini master and silicone brake fluid, I have a lot of travel on the pedal. I have bled the system many times and am convinced that there is no air in the system. The car is not done so I can't test it on the road. Any technical reason for the travel, like the master is too small for the bigger calipers?

Thanks, Animal

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Old 08-18-2006, 06:52 PM
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get rid of the silicone brake fluid and put in Dot 4. only advantage to the silicone is it is not hydroscopic, everything else is all disadvantages, especially the fact that it actully compresses to a point.
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Old 08-18-2006, 06:58 PM
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Did you completely drain and flush the entire system before putting the silicone brake fluid in? If not....you have problems....The two fluids are not compatible.
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Old 08-18-2006, 07:04 PM
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yup that's 1 more problem with the silicon dot 5 fluid, that's why I don't reccomend it. Speedway motors also warns agenst it's use.
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Old 08-18-2006, 07:33 PM
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Like henry says, If you change it flush it. If you go swapping fluids you need to flush the entire system with denatured alcohol. Something about mixing of those 2 type fluids swells the seals and makes everything feel spongy and have a low pedal. If the seals are swelled then they need to be replaced. You can't bring them back.

Is your pedal hard and low?
If so, your master cylinder may be too small. Are you using the entire stroke of the master cylinder? I usually set pedal/ MC up so that the brakes are applied fully at 1/2 to 5/8 of the piston stroke.
Hope this helps, mikey
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Old 08-20-2006, 07:22 AM
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brake pedal travel

Thanks for all the input. The system is brand new so there is not a flushing issue. There is no adjustment on the pedal linkage and there is only about a 1/4 inch of travel before the brakes are activated. The pedal travels about 3 to 4 inches before it stops about 2 inches off the floor.

I find it hard to believe that silicone can compress enough to be the problem. I have very little stainless braided hose - just from the frame to the calipers.

One suggestion that the master may be too small was a concern of mine. Is this a possibility in reality?
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Old 08-20-2006, 08:11 AM
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It could be the master cylinder size, but if the pedal moves 4 " and moves the piston in the master cylinder 1/4" and actually applies the brakes. then it is most likely your pedal ratio that is making the pedal low.

Are your measurements accurate? I question them because using those measurements your pedal ratio is 16:1. That is not a normal ratio.Most manual systems work well with a 5 or 6 to 1 pedal ratio, depending on the size of the master and calipers.
You said you are using the stock master cylinder from the mini.
What size is your master cylinder?
What is your pedal ratio?
Is the master under the floor or on the firewall?
later, mikey
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Old 08-20-2006, 08:15 AM
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Your master cylinder is too small . When you put the larger calipers etc. on your vehicle you should have changed to a larger master cylinder to compensate for the larger calipers.
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Old 08-20-2006, 10:06 AM
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brake pedal travel

OK. Actual measurements:

pedal off the floor: 4 5/8"

pedal linkage slack removed: 4 1/2" off the floor

pedal travel stop ( fully depressed): 2" off the floor

Pedal ratio: 5.5 to 1

These Minis have a small bore that is upright on a shelf directly above the pedal on the firewall. I don't know of anyone else who makes a master that mounts upward rather than sideways like normal.
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Old 08-20-2006, 10:33 AM
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It sounds like you may have some fabricating to do to get a bigger M/C in there.
The 5.5 pedal ratio with your measurements gives less than 1/2 of piston travel. Is that correct?

You will need a bigger bore MC to move the volume of fluid required to activate those calipers. Or a longer stroke MC.
is the current M/C a 3/4 bore single?
A dual M/C will move approximatly twice the fluid with the same stroke as a single. Your pedal effort will go up though.

I would put a gauge on your current system and see if you can attain 1000-1200 PSI .That car is pretty light, it should stop with those pressures. You can get a gauge set from speedway motors or stainless steel brake co. If you can't get that much pressure then changes are in your future.
hope this helps, mikey
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Old 08-20-2006, 11:11 AM
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brake pedal travel

Pretty technical stuff you have asked of me.

The master has 2 outlets, one for the front and one for the back. I have also added line lock and a Wilwood proportioning valve.

I kept the pedal/master setup just as it was originally so the amount of travel should be correct. I don't know the diameter of the piston inside the the master but I remember it to be about 7/8" (I have a similar one on my Healey).

There is no visible lowering of the fluid level in the reservoir when the pedal is depressed.
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Old 08-20-2006, 12:36 PM
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1) Is your master cylinder designed for 4 discs?

2) Did this MC work well before?

3) Do you have any residual valves in the MC or in the lines?

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Old 08-20-2006, 12:57 PM
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brake pedal travel

The master is new and there are no residual valves in the system,. The master is designed for discs all around at 8.4" rotors. I have 11" rotors in the back. The calipers are new (rear) and professionally rebuilt (front). I also stayed with the 3/16" diameter brake lines and the pads are up flush against the rotors with no pedal pressed.

BTW, the car is on jack stands and I have not driven it.
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Old 08-20-2006, 01:11 PM
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What were the piston diameter / number of the original calipers and what are the diameter /number of the pistons in the new calipers?

If the caliper piston diameters have been increased from stock, then you will need to increase the diameter of your master cylinder (somewhat) proportionatly in order to supply the calipers with fluid.
(The rotor diameter not really an influence when figuring the hydraulics, although a larger rotor will require less pressure to have the same stopping ability as a smaller rotor, with all else being equal)
Later, mikey
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Old 08-20-2006, 07:07 PM
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brake pedal travel

Mikey:

The front calipers when from 2 piston to 4 piston within the same size caliper so the total piston surface area should be less. The back when from drums to 1 piston calipers from a Fiero.

Although there is a lot of travel in the pedal (about 2 inches), it should stop the car alright, especially if I adjust the proportioning vale properly .

What do you think??
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