Hi

Can anyone help with advice on the best bore size for the master cylinder when running with Sierra disks front/rear on a Haynes Roadster?

The donor was a DOHC 2.0 and had ABS so I can't use that. I've currently fitted a standard Sierra 22.2mm cylinder, and despite bleeding the brakes multiple times I can still press the peddle to the bulkhead with my hands.

If it helps the front calipers are Teves and have 59.9mm pistons, the rear pistons are 42.8mm

From what I can work out the original master cylinder for this combination was either 25.4 or 23.8mm, but I'm not sure.

Also if you do know what size I should use, what master cylinder do I need to look at getting?

Thanks for the help

Matt

I suspect no-one will be able to give you a direct ‘right’ answer but there are plenty of threads on here about brake master cylinders if you search.

I don’t think a soft pedal like that is a spec problem though, surely there is something leaking or you have not managed to bleed it fully?

I've only just filled the system with fluid so I don't know if it's air or just a mismatch in sizes. I'm basically going on some other posts talking about going down in master cylinder bore diameter (from 22.2 to 19.05) as a way of firming up the pedal when using the disk/drum setup. That's an upgrade offered on the MK Indy site so I assume it makes a noticeable difference.

I'll do another round of bleeding the brakes anyway and fingers crossed that'll be enough - it's a lot less work than finding/fitting a new master cylinder and replumbing the brake lines

I've only just filled the system with fluid so I don't know if it's air or just a mismatch in sizes. I'm basically going on some other posts talking about going down in master cylinder bore diameter (from 22.2 to 19.05) as a way of firming up the pedal when using the disk/drum setup. That's an upgrade offered on the MK Indy site so I assume it makes a noticeable difference.

I'll do another round of bleeding the brakes anyway and fingers crossed that'll be enough - it's a lot less work than finding/fitting a new master cylinder and replumbing the brake lines

I normally adjust the rear brakes manually before bleeding and also check the bleed nipples are at the top of the callipers. If the nipples are at the bottom ( a common problem) you need to take them off and put a piece of wood in to stop the pistons coming out then turn until the nipple is at the top before bleeding. What pedal box are you using ? Pedal ratio?

The bleed nipples are at the top, but may not be at exactly the highest point so I'll take the callipers off and move them around a bit like you suggest.

I'm using the standard Haynes pedal box design, which has 31mm from pivot to where the clevis pin attaches and about 190mm pivot to centre of the pedal so I think that gives a ratio of about 6.1

I tend to use a pressure bleeder which makes the job a lot quicker and no need for an extra pair of hands ( or feet) a Gunsons Easibleed works well using pressure from a tyre and costs around £20 , they top up the reservoir as the fluid is pushed through the system , if you go that route make sure you have a maximum pressure of 20 psi. Better pressure bleeders are available , some people on here don’t like them but I have been using them for 40 years with no problems, I hate the vacuum type bleeders that some like.

Depending on where you have installed the brake light switch it might be worth just releasing it slightly to bleed any trapped air

[Edited on 5/7/20 by rusty nuts]

Thanks for the advice, I dug my old Easibleed out of the shed, and with a combination of that and removing the calipers so I could twist them and release any trapped air I could see I got rid of a few more bubbles and the pedal now feels a bit better. I also discovered a small leak in the pipework under pressure where I hadn't done it up tight enough and fixed that too.

I'm still worried about the amount of pedal movement as I can just about press it to the bulkhead, so does anyone know of a direct replacement master cylinder with a larger bore diameter?

http://brakepower.com/?fbclid=IwAR1h927vHeug25bnvqLgw80SJTtENXp20B91IC7jQnM96HB8hhUPhCDwB74

This might help

[Edited on 6/7/20 by rusty nuts]

Thanks for all the help.

I plugged the numbers into the brake calculator and it confirmed what I suspected, I needed a bigger master cylinder.

After a bit of trawling the web I found the Nissan Primera has a 25.4mm piston so I bought a master cylinder for one of those and fitted it.

The brake pedal is now a lot firmer and works how I'd expect, but I won't know for sure until I get the car running.