V8 Brake Master Cylinder Upgade to Larger Bore

[dclare]

This may not be relevant to others with big brake conversions because its all dependent on the caliper piston sizes but I thought I’d document in case anyone has a similar issue.

 My car is fitted with 6 pot Alcon fronts calipers and 4 pot AP rear calipers which provide fantastic stopping power and resilience, but the pedal travel with the standard master cylinder was very long and overly light, so the only answers to giving a shorter and heavier pedal were to either change the pedal ratio or fit a larger master cylinder. I didn’t fancy messing about with the pedal box so I started the search for a master cylinder that would be as close as possible to the original but with a bigger bore.

 The standard Bendix master cylinder is 23.8mm bore and after a few false starts I found a VW part with a 25.4mm bore from a company called Retrofication – so 13.9% greater piston area.

 https://www.retrofication.co.uk/vw-mk1-mk2-mk3-golf-254mm-brake-master-cylinder-16v-vr6-18t-tdi-brake-upgrade.html

 When the part arrived it was manufactured by Topran, Part Number 110 041 – so pretty freely available on-line. The cost from Retrofication was £49.50, so relatively inexpensive…

Examination of the two parts side by side looked pretty good, however the ports on the new cylinder are M12 rather than the standard M10 size, however the servo mount and reservoir ports/mount are perfect.

This meant changing the union on the end of the brake pipes from the ABS block….easier said than done! After cutting off the old one I tried to fit a new union with the pipes in situ, but there just isn’t enough room to get the flaring tool in properly and there isn’t really enough length in the pipe, so I decided to remove the pipes and make up a new ones altogether. What a ball-ache it was getting the old ones out of the ABS block! Without removing all the unions from the block it is very hard to get onto the necessary pipes, and the unions themselves were very tight/semi-ceased.

 

The new unions fouled the black plastic relay cover so this was reshaped with the Dremmel and once fitted at all looks pretty OEM, so I’m happy with the result.

 

Once everything was reconnected it was simply a matter of double checking everything re-filling with new fluid and bleeding all the brakes and checking for leaks.

 

 Conclusion

Definitely a worthwhile change for me. The brakes are far more intuitive now and much easier to modulate. In fact I stopped thinking about them after a few miles, so they definitely feel better. Its also much easier to heel and toe now because the pedal works so much higher.

PS - why do photos make everything look so dirty!! 

 

 

[Erikl]

Brilliant.  Excellent work.  I have been thinking of something similar and I have wondered if I could find a standard master cylinder with just minor changes to the current unit.

 

I would love to change out to twin Masters with a balance bar and no vacuum servo but I dont want to go nuts fabricating and cutting.  Especially in the pedal box area. 

[mike_sekinger]

Great write-up Dave.

Locally sourced for the grand total of 31.58€ incl. shipping.

[Erikl]

Why did this work?  I am trying to remember my fluid mechanics.  Intuitively I think this would give a softer pedal but a shorter stroke - cuz its moving more fluid but the pressure is reduced with the larger area......But that does not really make sense. 

Why did a larger cylinder give a firmer pedal?  

Is it just the shorter stroke makes it feel firmer.  

thx

[drdoom]

Basic summary on this is that hydraulic advantage ( think fulcrum point in a lever situation ) reflects the area ratio of master-slave cylinders, wherein the smaller the M/C the greater the line pressure from a given input force at the pushrod. Example: 400 lb. at the pushrod into 1" area = 400psi, whereas 400 lb. at pushrod into 2" area = 200psi. Similarly, any stroke arising from yield in any points within the entire brakes system, including physical movement of calipers, will require greater stroke at the pedal with smaller bore M/C due to fluid volume required to take up physical displacements. Dave's outcome is entirely consistent with the theory. 

Cheers

[dclare]

Yep, in layman's terms a smaller master cylinder means the pressure from your foot is concentrated into a smaller area, therefore higher pressure at the piston. A larger cylinder means more foot pressure is required to create the same pressure at the piston. Also if the pedal travels the same distance with the larger cylinder it will move more fluid. So, the summary of both effects is more force required at the pedal and less movement to push the same amount of fluid.