Fiat Dino Forum (FDF)

After fitting a new master cylinder and fully reconditioned brake servo/booster, whilst out on a shake down drive,the pedal sometimes travels much further down past the usual position, then on the next use of the pedal it's fine.
All brake calipers have new pistons and seals and worked fine before. There are no leaks and the fluid reservoir are unaffected.
Does anyone have any ideas.

As nobody has answered so far - its difficult to give a diagnosis from remote.

Some ideas from my side: This behaviour sounds as if there is still some air somewhere in the hydraulic system. So another thorough bleeding might be a next step. Depending on how you bleed make sure the brake proportioning valve does not interfere/influence your bleeding and (don’t laugh I have seen this more than once) the bleeding valves are on top of the calipers / the calipers are not interchanged left-right…

KR Thomas

Bad master? I know you mentioned it's new.

Not sure what's causing you the trouble, but it happens.

Hi, I suspect you have an issue with internal bleeding in the brake master.
One of the gaskets in the brake master are leaking/ not sealing properly, and causing internal bleeding.
This is typical recognized when the brake pedal experience you describes.
If you "shock" the brake pedal, by stamping it quick, the pedal becomes normal?
This is caused by the gaskets on the piston not sealing properly, but if you "shock" them, the brake fluid will force gasket lips out and they will seal properly for a while.
When the brake pedal is depressed slovly, the brakefluid leaks past the gasket (causing internal bleeding in the brake master), and the pedaltravel becomes longer!

BR
Erik

I suspected it was the seals in the master cylinder so disassembled it. What I found was very surprising,six raised bobbles on the first seal only closest the booster. My original seals were not like this and nor were the replacement seals from pastparts.These bobbles would actually stop the seal sealing effectively.

Forgot to add image