Brake pedal pressure

[kiwi ka]

As I work up all the vital aspects of my Mclaren (oops McCopy) I have made a change to my brake cylinders. I have very good strong responsive brakes with a soft pedal. There is major work to change the ratio (6 to 1) and I was running 3/4 master cylinders and wanted 2 things ...less travel and more pedal feel / hardness/ pressure.
I changed to 7/8 cylinders and have not experianced any real change.
Any clues.
Cheers
Russell

 

[Russ Noble]

Yeah!

Are you running 'rubber' flexible lines or aeroquip type braided stainless steel reinforced lines? If rubber go to the s/s braided. If s/s braided go to yet bigger m/cyls.

Glad you are getting into the 'fine tuning' details. I am envious of your progress.

Cheers,

Russ

 

[Russ Noble]

One other thing Russell, check that your caliper mounts are parallel to your rotors. If not you will have increased pedal travel and some softness as the calipers/pistons/pads distort to take up the excess clearance/misalignment.

 

[Randy V]

Russell - It sounds like she's still got just a bit of air in the system... I've had some that were just a bear to get totally clear..

One thing that seems to help is to PING the calipers or mounting bolts with a small ball-pein hammer 5 or 6 times or so before each "easy" stroke of the brake pedal. This has the benefit of shocking any clinging air-bubbles to the top where they can be bled from the system. This is all presuming that your bleeder valves are all at the very tops of your calipers..

 

[Phil]

Russel:
Suggestions from Russ and Randy are very relevant. A 3/4 master cyl, with a 6 to 1 pedal ratio should produce roughly 1360 psi., a lot of pressure and I can see why this would cause a spongy pedal. A jump to 7/8 master cyl. brings you down to approx 1000 psi., a 27% drop in pressure...you should definitely feel a change in pedal firmness here unless you have a problem with air or line expansion as was mentioned.
I am assuming you are running a balance bar, and you might have a problem with one part of the system, front or rear. You may need to isolate them and check which is the culprit.
I would guess that going to 1" masters may not give you enough braking pressure, but not knowing the size of your caliper pistons it is hard to hazard a guess.
I would take the two recommendations from Russ, and Randy and make sure you have all the air out of the calipers, and if your bleeders are not in a top position you may need to move them to a position to acheive that, then re-set them where they are mounted.
Hope this helps
Phil

 

[Graeme Stebbing]

Hi Russell, you might find the answer to you problem lies somewhere in these pages definately worth a good read, there is 7 pages of handy info here.

Tech Article on Brake by Dean Oshiro

Best Regards
Graeme

 

[Cliff Beer]

Russel, are you pads fully bedded in? It can take a good 500+ miles to properly bed the pads in. If not fully bedded you'll experience a little bit of softness, perhaps not quite all of what you're feeling there but possibly part of it.