Brake Troubleshooting

[jezza323]

I have converted the rear brakes from drums to disks, and upgraded the front to larger disks and calipers with a very slightly larger piston.

I have bled the system, and i get good pedal pressure/height, UNTIL i start the car, then the pedal goes completely shit and hits the floor. once i turn the car off i can get the good pressure back with 2-3 pumps of the pedal.

any ideas what is going on?

only thought i have so far is the master cyl is too small? but then why can i get good pressure when the car is not running (no vac boost)?

[Cuzzo]

Im pretty sure that when starting the car with your foot on the brake it is meant to go down further but not to the floor.
Did you bleed the brakes correctly and get all air out of the system?

[jezza323]

yes im confident there is no air in the system, bled brakes at least 5 times before...

[Jasper]

Sounds like you are confusing 'good pressure' with a higher force per unit travel without the vacuum assistance.

Using a larger diameter master cylinder will fix the problem you have with excessive pedal travel.

[jezza323]

as soon as i am able (ie tomorrow morning) i will find out the master cyl bore of my current master, and of the master for the car that the brakes came off....but as far as i know they are the same...

[Jasper]

The geometry of the pedal setup may be different though. Check the distances between pedal centre, booster input shaft and fulcrum on both setups. This has just as much influence on the system as the MC bore.

[jezza323]

its an EP91 starlet vs an EP82 starlet, the chassis/floorpan is almost identical, as are most parts...i cant see it being too different

but its sounding like i will need to take the car to a professional to get it sorted out anyway...

[Jasper]

Do I count? I'm a design engineer, currently working at an aftermarket brake manufacturing company

[jezza323]

yeah u do, but u r on the other side of the country

im supposed to be attending a sprints event on saturday...australia day friday...gives me 2 days to work it out...

any chance u can look up the master cyl bores for me?

[Jasper]

The bore size is usually stamped on the MC. What have you got in there now? What were the piston sizes of the old calipers? What do you have now? How many pistons? Wheel cylinder size of old drums?

[jezza323]

was all std aussie delivered EP91 stuff, i dont know the details...

new pistons are 51mm, think old ones were 48mm? all single piston calipers

[Jasper]

Assuming the pedal ratios on the two cars are the same, using the MC from car which donated the brakes should fix it.

Is the bolt pattern of the two MCs the same? Do they both use the same diameter booster? Are the boosters both single or twin diaphragm?

When changing master cylinders, you need to adjust the clearance between booster output shaft and MC piston. This is done by checking the distance with a special tool (or verniers), and turning the acorn headed bolt on the booster output shaft.

[TA-022]

WOW technical.

I'd just fit the booster and MC from the 91.

[jezza323]

im getting a GT master cyl and booster on friday, i will have that fitted in the next couple of weeks and see how it goes...

[Negative Boost]

Yup, i agree to that your MC is too small.

I was using a 7/8ths MC before i upgraded my brakes to 4 spots on the front. After that i had the exact same symptons as you. Bleed the brakes heaps, good feel when the engine was off but crap, soft, almost hit the floor when the engine was running. Swaped the MC to a 15/16ths and the brake pedal has dramatically improved. Its got better feel and dosn't travel as far now. Still not optimal though (want a better initial bite, still little too soft for me), have to try a 1inch bore.