Brake pedal to the floor

[Adsie]

Car - AE86

Motor - Blacktop 20V

Use - Track

Problem - Brake pedal to the floor

Occurs/started when - After motorkhana run

Personal situation - At the last motorkhana after each run (about 20-30 sec flat out 1st/2nd gear) when trying to stop the brake pedal would go straight to the floor. Anyone shed some light on whey this could be occuring? Car has front and rear JDM disks with all rubbers lines replaced with braided ones. Master cylinder was rebuilt about 2 years ago and you can pump up the pedal with the car off and it feels soild. So I am thinking it has something to do with the vaccum. On the blacktop one of the original vaccum lines for the booster is used and just ran a rubber hose from the front of the motor while the other one is blocked off.

Thanks

[hachi_dk]

sounds like master O ring may be gone?

[Adsie]

Thought if the master cylinder o ring was gone it would happen all the time? This seems to only happens after sustained
high revs.

[H8CHIR6KU]

nah your setup sounds ok. im running the same and its sweet. how long ago did you replace the hoses to braided.

i always recommend replacing or rebuilding the master when changing to braided. the extra pressure that the braided hose holds usually ends up blowing out seals in the master causing it to bypass when pedal is applied.

i dont think thats whats happening, the only thing i can think of is that you dont have the one way valve fitted or its the wrong way around

[Sam-Q]

if you where running out of vacuum the pedal would be hard not sink down

[Adsie]

The braided hoses were done about 6 months ago. Is there a way to test the master cylinder besides what I have already tried? There is a one way valve fitted, will test that tomorrow make sure it's working.

[lolwat]

sorry if already said but does it only do it after long periods of heavy on and off braking, or all the time

[Adsie]

Pedal kind of always feels crappy but has only gone to the floor after a good thrashing. Like after a motorkhana run and once at the end of the straight at lakeside. So the brakes had time to cool when this occurred

[ke70dave]

yer i would still be looking at your master cylinder.

my girlfriends car had a simliar problem, the brakes would work great most of the time, but every now and it would just go straight to the floor. turned out to be the mastercylinder

this problem may be exadurated at a motorkhana since you are hitting the brakes harder and more often etc.

and if your calipers/slave cydliners and all your lines arent leaking, then i dare say there isnt much else it can be.

as stated the vacumme is only there to make it easier, your brakes should still work perfectly fine without the booster (just a tad harder to push..)

[lolwat]

if it after a thrashing, why not try as a cheap option doing a full brake flush and using new fluid, as it might be boiling

my bosses ute looses brakes after he thrashes when works turning to shit