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Master and slave cylinder relative sizes
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colwyn500



Joined: 21 Oct 2012
Posts: 1745
Location: Nairn, Scotland

PostPosted: Thu Sep 18, 2014 8:40 am    Post subject: Master and slave cylinder relative sizes Reply with quote

I have had some comfort in a positive answer on another forum Embarassed referring to my specific car, but would appreciate having the benefit of collective wisdom.
The car has a straightforward, single circuit brake system with (as original) equal sized cylinders at and drums on all four brakes.
The replacement cylinders are as original at the rear but about 25% larger in diameter at the front. It all fits and seems to be accepted as workable and safer than the original. The niggle I have is that my basic understanding of physics means that I am concerned that the pedal will need to travel further in order to operate the brakes.
Are there any opinions or experience on this subject please?
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baconsdozen



Joined: 03 Dec 2007
Posts: 1119
Location: Under the car.

PostPosted: Thu Sep 18, 2014 9:22 am    Post subject: Reply with quote

The pedal would have to move a bit further,but if the brakes are adjusted properly the increased travel can be kept to a minimum. Brake systems have enough 'spare travel' to allow for badly adjusted/well worn brakes so if you keep yours in good adjustment you should be OK. I suppose you could check by deliberately setting the brakes badly (with excess play) to see how much 'reserve' has been designed into the system.
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Thirty years selling imperial hand tools for old machinery(Now happily retired).
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Old Wrench



Joined: 23 Dec 2013
Posts: 226
Location: Essex and France

PostPosted: Thu Sep 18, 2014 9:47 am    Post subject: Reply with quote

Firstly, here are some references.

Larger wheel cylinders would increase necessary fluid "Displacement": therefore, pedal travel would increase.

However, of more concern would be Brake Balance, front-to-rear. Mainly since by fitting larger cylinders, but maintaining the design hydraulic pressure, the force generated will be across reduced surface area.

Perhaps the best personal illustration of this to me, was a tweaky Cooper 997 c.c. I purchased and one evening I was going from the main garage to the bodyshop, about two miles away. It was November, damp and cold.

Arriving at my, shall we say of those days, hurried pace, I wanted to turn a 90 deg left junction: and a front wheel locked, violently and I continued straight ahead, but er, rather sideways!

Next morning in the workshop I jacked up the front and removed the drums. To discover some bright spark had fitted early Mini cylinders on one side; and later Cooper cylinders on the other!

(Edited to remove stupidity! The first Mini-Cooper 997c.c. did in fact have disk brakes. These were worse than decent twin leading shoe (early Mini 850s had single leading shoe) drum brakes. Therefore when tweaking it was common to revert to drums. There was a wide choice of back plates and cylinders. Quite a bit about wheel cylinder sizes here, too:
http://www.mokeclub.org/forum/viewtopic.php?f=9&t=5038&sid=45b9f1142e5b878cd472e54a4c5885d6&start=15 ).

Going at normal speed never a problem: under heavy braking on a damp surface, lock up.

New identical wheel cylinders both sides cured this immediately.

http://enderw88.wordpress.com/automotive-theory/brake-system-theory/

Pascal's Principle

"Pressure is transmitted undiminished in an enclosed static fluid."

http://www.calverst.com/articles/brakes-rear_brake_bias_adjustment.htm

http://www.pirate4x4.com/tech/billavista/Brakes/#HowBrakesWork


Last edited by Old Wrench on Thu Sep 18, 2014 5:20 pm; edited 1 time in total
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colwyn500



Joined: 21 Oct 2012
Posts: 1745
Location: Nairn, Scotland

PostPosted: Thu Sep 18, 2014 1:40 pm    Post subject: Reply with quote

Top answer Young Mr. Spanner. thanks Laughing
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