Technical A quick question about brake cylinders.

[Thomas Vaughan]

I'm going to buy four new brake cylinders but when I look at the listings it states different size bores?
19.05ø, 23.8ø and 15.9mmø
When I look at the existing cylinders they appear to all be the same size front and rear.

What am I measuring?

1998 elx

 

[bugsymike]

I'm going to buy four new brake cylinders but when I look at the listings it states different size bores?
19.05ø, 23.8ø and 15.9mmø
When I look at the existing cylinders they appear to all be the same size front and rear.

What am I measuring?

1998 elx

Generally if you remove the dust cover from a brake cylinder and very carefully have someone pump the brake pedal until the piston protrudes from the brake cylinder just enough to put a vernier gauge or similar around the piston you should get a reading that compares with one of those new cylinders available.
A simple guide if no vernier available is to slide a metric open ended over the piston i.e. 16mm,19mm or 24mm should tell you what size.
Obviously a correct match is best, but failing that it must be the same on each axle for balance. What you would notice if too big or too small is down to the principles of hydraulics and the pedal effort versus pedal travel required.
In the old days the cylinder diameter was affected by whether there was a brake servo fitted or larger brake drums on your specific model.
Another thing that can effect choice of brake cylinder is the make as some vehicles used different makes,ATF, Lockheed, Girling,Bosch etc. All used different mounting points.
Another small point do not let assistant press piston out too far or you will be without working brakes for longer and brake fluid all over the brake shoes

 

[Thomas Vaughan]

Generally if you remove the dust cover from a brake cylinder and very carefully have someone pump the brake pedal until the piston protrudes from the brake cylinder just enough to put a vernier gauge or similar around the piston you should get a reading that compares with one of those new cylinders available.
A simple guide if no vernier available is to slide a metric open ended over the piston i.e. 16mm,19mm or 24mm should tell you what size.
Obviously a correct match is best, but failing that it must be the same on each axle for balance. What you would notice if too big or too small is down to the principles of hydraulics and the pedal effort versus pedal travel required.
In the old days the cylinder diameter was affected by whether there was a brake servo fitted or larger brake drums on your specific model.
Another thing that can effect choice of brake cylinder is the make as some vehicles used different makes,ATF, Lockheed, Girling,Bosch etc. All used different mounting points.
Another small point do not let assistant press piston out too far or you will be without working brakes for longer and brake fluid all over the brake shoes

Thanks Mike for the extensive explanation!

So it doesn't matter as long as they are the same on each axle.
Would different sizes front and rear act like brake bias?
I have Vernier calipers, I'll measure.
I'll most likely just replace like for like though.

 

[bugsymike]

Thanks Mike for the extensive explanation!

So it doesn't matter as long as they are the same on each axle.
Would different sizes front and rear act like brake bias?
I have Vernier calipers, I'll measure.
I'll most likely just replace like for like though.

Yes, best to do like for like.
Any different bore size on same axle would be dangerous.
In the old days there were many brake cylinders that looked the same on the outside , but internally bore was different so could be dangerous. I remember on Mk 1 Ford Transits there were many choices as they had different load capabilities.
It was all before ABS etc. and often single circuit brakes so one pipe from the master cylinder, meaning if any area failed you had no brakes at all!
Different front to rear would affect brake bias , but also I suspect your car has some form of brake limiter to the rear also.