Brake pedal is spongy but still works well

[als804]

My brake pedal got suddenly weird. When i drove trails I pushed the brake pedal many times and then clearly noticed i got spongy pedal. It’s very very sudden and scary. But the brake still works……

but there is no resistance until reaching to 3/4 travel of The pedal. there is no any fluid leak in car and master cylinder reservoir still has full of fluids. But No brake pedal pressure but i can feel pressure at almost last point where Brake starts to work. And i tested the brake that i was putting both brake pedal and gas pedal together then car didn’t go. I can’t push the brake pedal to floor due to pressure And i noticed there is still brake power at the last point.

is this master cylinder failure? Power steering works nice

 

[als804]

It looks like “internal leaks” i will take a look at it tomorrow. It’s lucky i already have a brane new master cylinder !

 

[adf5565]

I’m not sure this would happen suddenly but I had a similar issue after replacing my brake system. Turned out it needed to be bled more as there was still a pocket of air in the system and also my rear shoes needed to be adjusted closer to the drums. It’s worth checking while you’re troubleshooting.

 

[als804]

the next day i tested my brake and worked it again. one day resting could make brake fixed ?
i guess internal seals problem in the master cylinder. so i decided to replace it with brand new master cylinder which i already has.
changing it is very easy jobs. i followed the bleeding procedure in TM. also i found input rod of master cylinder was very rusty and dirty. so i cleaned it with wd 40 and sprayed Sillicon oil. i used DOT 4 Brake oil instead of DOT 5 wich you can see it with very rusties in the master cylinder below picture.
( Choosing breake fluid is always battlefield. In Korea is very humid so i will regulary change DOT 4 which can abosrber water much more than DOT 5. )
Right after chaning it i can feel very soft and strong pressure of brake. i love it,










the old master cylineder which i suspected internal leakage.

 

[Keith_J]

Old master cylinder had DOT4?

 

[als804]

Old master cylinder had DOT4?

Old has dot5
I use dot4 on new master cylinder

 

[Keith_J]

DOT4 and DOT5 are not compatible, you will have problems soon as these gel when mixed.