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Thread: Pedal box master cylinder sizes - for standard & uprated brakes

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    Spanner Monkey the1theycallthe1's Avatar

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    Pedal box master cylinder sizes - for standard & uprated brakes

    What size master cylinders would i need for a pedal box if:

    A) The brakes were standard - M16 fronts & rear drums

    B) Wilwood Dynalites up front & cossie calipers at the rear

    Cheers!

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    Re: Pedal box master cylinder sizes - for standard & uprated brakes

    If its a single cylinder box then it would be

    A) 0.70"

    B) 0.75"

    If it was a bias box with separate front and rear then it would be

    0.625 front

    0.70 Rear

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    Re: Pedal box master cylinder sizes - for standard & uprated brakes

    as per Neil except i say with bias box go for 0.75 rear

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    Re: Pedal box master cylinder sizes - for standard & uprated brakes

    Quote Originally Posted by the1theycallthe1 View Post
    What size master cylinders would i need for a pedal box if:

    A) The brakes were standard - M16 fronts & rear drums

    B) Wilwood Dynalites up front & cossie calipers at the rear

    Cheers!
    If you go for a bias pedal box (no servo) and Wilwood Dynalites at the front set the bias ma. towards the front and be prepared that you need to press the pedal very hard, especially if you use normal-ish pads.

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    Re: Pedal box master cylinder sizes - for standard & uprated brakes

    Thanks, should have mentioned it was a bias box.

    so with either set of brakes im looking at 0.625 front and 0.75 at the rear?

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    Re: Pedal box master cylinder sizes - for standard & uprated brakes

    Bump to this thread for another question. Does the size of the cylinders make any difference to the pedal feel?

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    Re: Pedal box master cylinder sizes - for standard & uprated brakes

    yes, the smaller the cylinders the longer the pedal travel and less pedal pressures needed

    bigger cylinders give a more solid feel and less travel to the pedal but you have to push harder

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    Racer Decade Plus User wildo105e's Avatar

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    Re: Pedal box master cylinder sizes - for standard & uprated brakes

    Thread revival time. I am having a nightmare with bleeding / setting my brakes up.

    So here's the scenario.

    bias pedal box with 0.700 front master cylinder with m16 calipers. 0.625 with rear drums for now. The issue is i cannot get any braking effort out of the rear. I don't think air is the issue after the amount of bleeding i have done. I have bled traditional, way air powered suction and let fluid just run through with gravity etc.

    I used these sizes as i copied my milton box on the anglia and this works great with rear sierra calipers.

    Quote Originally Posted by neilm View Post

    If it was a bias box with separate front and rear then it would be

    0.625 front

    0.70 Rear

    Reading the above sizes have i got the cylinders mixed up ?? and also does it make a difference to which side of the pedal the cylinders are ? My front is on the left and rear right side nearest the door. Maybe i just need to swap the pipes over so i have .700 rear and 0.625 front (but the fronts feel ok as they are)
    or is the rear just to small to power the rear drums ?

    cheers
    Last edited by wildo105e; 17-06-2018 at 16:43.

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    Re: Pedal box master cylinder sizes - for standard & uprated brakes

    Yes - you seem to have them back to front - smaller dia to front for greatest pressure and depending on how the bias box is constructed the position can have an effect. Mine has a pivot bias across the bias bar ie. the cylinders aren't spaced equally so each gets differential pressure from the pedal - don't ask me which is which as I didn't notice when I took mine apart LOL!

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    Re: Pedal box master cylinder sizes - for standard & uprated brakes

    think i will switch them around so the pipe work still fits and see if that helps

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    Re: Pedal box master cylinder sizes - for standard & uprated brakes

    Size of master cylinder should not matter when bleeding brakes.
    Do you have a hydraulic handbrake?

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    Re: Pedal box master cylinder sizes - for standard & uprated brakes

    Thanks all so far for the advice, removed the box as it was easier than trying to swap in the car. This is the setup looking at the pic (upside down) the clevis on the left is spaced away from the pedal by a fabbed spacer etc, whilst the one on the right isn't. I assume this gives less pressure on the left and would hence be the rear cylinder.

    Is that a fair assumption ? if so i had it right way round just the size of the cylinders reversed.

    cheers

    Last edited by wildo105e; 22-06-2018 at 22:07.

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    Re: Pedal box master cylinder sizes - for standard & uprated brakes

    If you truely want to understand the best front and rear master cylinder sizes you need to use maths and not rely on old wives tales (note - all size suggestions above do not reference the caliper/cylinder sizes so I do not know if they calculated their answers or not so this is not a dig at anybody. The combo of parts you are using is probably well tried so people probably just know what works). My tarmac rally car had good brakes when I built it......until I actually calculated the optimal size M/C's and now it has so much better braking!

    As for your rear braking effort or lack thereof.....if you have drums on the rear, my understanding is that the rear slave cylinders have to travel alot further than a disk caliper piston due to design. If you currently have such a small M/C for the rear I would expect that the M/C either bottoms out before the rear cylinder if applying enough pressure or worse is just not travelling anywhere near what it needs to. As I said though, you need to measure and calculate. I had a similar issue on an old Mustang.

    I cannot recall what size cylinder the rear drums on an Escort use but lets say 20mm slave bore and your 0.625" M/C. The MC has an area of 198mm2 whereas 1 rear slave cylinder is 314mm2 but you have 2 so 628mm2. If the MC moves 20mm (a complete guess based on pedal movements and pedal ratio and probably too high) then the MC pushes out 3,959mm3 of fluid. This in turn moves each rear slave 6.3mm which may not even be enough to take up the slack in a drum setup => no pressure applied between shoe and drum. (Note: just come off nightshift as double check my maths!!!!)
    Last edited by MK1_Oz; 23-06-2018 at 00:45.
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    Re: Pedal box master cylinder sizes - for standard & uprated brakes

    Bit simplistic but basically sound info except there is not much more movement from a drum braked shoe compared to a disc pad - IF ADJUSTED CORRECTLY. If retaining the rear drums, then IMO, there is no reason not to have a bias separated master cyl. that is not the same size as the original single master - pressure will be identical so should work the same?

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    Racer Decade Plus User wildo105e's Avatar

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    Re: Pedal box master cylinder sizes - for standard & uprated brakes

    cheers guys, i think you have confirmed my suspicions that it was correctly bled i was just not shifting enough fluid.

    Watch this space

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    Re: Pedal box master cylinder sizes - for standard & uprated brakes

    Ok guys on further investigation it turns out my rear cylinder was 0.75. Girling cast it so badly it was hard to read. Now replaced with a 0.7 and that bled up fine. Had trouble again with the fronts so i have drained the rears as I wasn’t getting enough travel to shift fluid in the fronts??? They finally bled up but I have now have a random situation where the fronts are bled and work fine (no fluid in the rears) but the pedal will randomly just drop to the floor, pump it 4 or 5 times and they pressure up again.

    I can also replicate this by pulling the brake pedal towards me? Same again a few pumps and all is restored, I don’t fancy driving the car like that.

    I am assuming this isn’t right even with the back brakes open to air I should get constant pressure to the fronts before I bring the backs into the game.

    I have no leaks that I can fine so air isn’t getting back in. Am I just unlucky and also have a dodgy Master for the front??

    Any thoughts appreciated

    Thanks again guys

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    Re: Pedal box master cylinder sizes - for standard & uprated brakes

    thats weird, you probably have a knackered front cylinder. in my experience you wont get a decent pedal until both circuits are bled up

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    Racer Decade Plus User wildo105e's Avatar

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    Re: Pedal box master cylinder sizes - for standard & uprated brakes

    Quote Originally Posted by Graham View Post
    in my experience you wont get a decent pedal until both circuits are bled up
    Thanks graham, I currently get half pedal pressure when it works on the front , which i would expect then get the random issue. I am pretty sure i will get good pedal pressure if i bled the rears. I am just wondering for 5 minutes of bleeding the rears if that makes a difference to the fronts. I would just need to do a lot of testing to be sure as i suspect the rears may mask the fronts. Dont fancy hurtling into a bend with only rear brakes would make a great Scandinavian flick

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    Re: Pedal box master cylinder sizes - for standard & uprated brakes

    ok i have just bled the rears and have the issue on the fronts. Which won't go away as i cannot pump the fronts enough to clear the issue as the rears are locking up (as they should) preventing further pedal pressure.

    I think for the sake of a new MC i will replace it.

    Not sure i am can trust it now anyway.

    Oh well out with the pedal box again.
    Last edited by wildo105e; 07-07-2018 at 13:11.

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    Re: Pedal box master cylinder sizes - for standard & uprated brakes

    If you have any dirt in the M/C it can stop the plunger blocking the fluid inlet port (when you push the pedal it pushes the seal down the M/C but also pushes another seal against the rear wall of the M/C). When this happens and you press the pedal the fluid can fully or partially flow back into the reservoir thus giving little to no pressure. I have had seals start to perish in the past and the bits of rubber held the port open.

    You can test this theory by having somebody give the pedal a big shove whilst you look into the reservoir. If what I think is happening you will see the reservoir fluid surge or move. = M/C rebuild/replacement.
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    Re: Pedal box master cylinder sizes - for standard & uprated brakes

    Quote Originally Posted by wildo105e View Post
    Am I just unlucky and also have a dodgy Master for the front??
    well after an age i finally got round to getting the box back in with a new 0.625 cylinder for the front and the brakes bled up quickly (well practiced now) and all looks good.

    At least i can move on now.

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