Hadley71

Lexus LS 400 1996

lexsmooth

possibly leak in the system. checked fluid level, if low. find source. check lines especially the fitting to calipers. or possibly caliper piston seal...

spuds

If not losing brake fluid its time for a new master cylinder or rebuild.

RA40

iTrader: (6)

Vacuum leak if not losing fluid. Brake boosters are common also, have that checked.

dicer

Air is system ? Is it trac?

Hadley71

Losiracer2

Its your internal seal on the master cylinder. I had this happen on my Accord, it would have good initial brake pressure, but as you held it for a period longer than 5-10 seconds, it would sink to the floor as the fluid was getting bypassed by the internal seal.

spuds

Its one or the other.

fixmiester

Only two possibilities; fluid is either leaking inside the master cylinder internally, or you are losing fluid via an external leak, which is usually visible somewhere. Your symptoms point directly at the master cylinder internal seals. There are rebuild kits available at very low cost, for those who know what they are doing. BTW, the vacuum booster only assists the driver in pushing the pedal down; it does not cause the loss of pressure you are experiencing. When it fails, you lose that assistance, but the brakes still work fine.

timmy0tool

iTrader: (7)

if there is no visible leak, the master cylinder itself has bad seals and is allowing fluid to pass through, as losi stated. this is the case if you pump and the pedal pressure restores.
sometimes the MC leaks INTO the booster so it's hard to see any leak at all. replace that MC!