Brake master cylinder mystery

[Certified Lotus]

I replaced my brake master cylinder with a correct type for the S1 Elan. The brake pedal is very hard and after driving for 10 minutes with minimal brake use the front brakes start to slowly lock up.

I have bled the entire system multiple times and even removed the master cylinder to inspect the valve and seals.

The only way to release the front calipers is to crack the fitting on the master to releave pressure, but it happens all over again after a few miles.

Any suggestions regarding the potential problem?

[mbell]

I'd guess the valve seal isn't releasing pressure/fluid back to the reservoir.

Things to check are the seals and springs. My guess would be the length of your push rod thou. My plus two needs one that's shorter than pretty much any that come with the masters.I believe the elan uses the same setup/length.

Did you fit the one that came with the car our the one that came with the master?

Maybe post measurements if your push rod...

[Certified Lotus]

I did use the same push rod that came with the car. Although the car came with a tandem master cylinder and I replaced it with the original single master cylinder. I wonder if the rod is too long. I'll need to check that. Good suggestion. Thanks!

[Andy8421]

I have had the same problem myself. When the pedal is released all the way, there needs to be a small amount of free play in the pushrod. This is to allow the piston in the master cylinder to return all the way to the retaining circlip at the entry to the cylinder. Just before the piston hits the circlip, the rod in attached to the piston inside the cylinder pulls the non-return valve away from the end of the cylinder uncovering a hole to the storage reservoir, releasing any residual pressure remaining in the cylinder.

If the piston doesn't come back far enough, the hole remains sealed and any residual pressure in the system will remain. If the brakes have just been used, thermal expansion of the fluid as heat bleeds into the callipers from the pads and disc will have no where to go and will increase pressure in the system causing the brakes to lock up.

[Certified Lotus]

Does anyone have the exact length of the brake master cylinder push rod for an Elan S1 ? I'm trying to determine if mine is too long.

Better yet, does someone have an OEM one for sale?

[mbell]

Based on:
lotus-gearbox-f37/clutch-master-pushrod-lenght-t15030.html

~73mm.

[alan.barker]

i had this problem on an Elan and i fitted some shims between the master cylinder and the bulkhead to give some free play in the piston rod
Alan

[Certified Lotus]

To close out the discussion, I removed the 4" pushrod (measured end to end) and installed a 3 1/2 " pushrod. Works like a dream!

Thanks for everyone's advice and counsel!