At the beginning of a session my brakes start soft. Over the course of the session as I brake later and later and drive more and more they get really hard.

I drove my friend's Integra and his brakes start hard and end hard after a session.

My other friend also commented on my brake pedal feel.

I've switched from Generic DOT 4 to RBF600 to ATE Gold and it still didn't fix the issue.

Here's my setup:

• Integra front and rear brakes
• 91 Civic EX Brake Master
• Stock proportioning valve
• Stainless lines

I've bled the system multiple times and this is still an issue.

Thanks.

 

vacuum issue?

 

Dunno. Smoke test booster lines?

 

is the brake master new? how old are the components? or do you have any idea'r on the mileage on them (assuming theyre not new..)

did you do a bench bleed?

 

brake master is from 2015 and was purchased new. Only used on track. Calipers were off an integra but no idea mileage. Prop valve is original to chassis which is ~200k miles i think.

I bench bled the MC but maybe I made a mistake. Woudl that trap air in the system?

 

apparently..if it is not done correctly air can be trapped in the system.

no idea if a prop valve could cause this or if old calipers might..i suppose you could try swapping the calipers off your friends car to see if they're the culprit?

 

throw a master in it just to be safe. last year before i got the stoptech kit i had weird inconsistencies in the pedal occasionally, and it seemed the rear seal on the master was leaking when i popped it open

also do yourself a favor and buy a MC brace if you dont have one. made a HUGE difference in my car. i had almost an inch of firewall deflection.

 

standard RS/LS/GSR calipers all the way around? What size is the MC?

 

Thank you all so much for the replies. They're really great!

Probably not gonna swap calipers as I think the issue with the master cylinder is a probable cause. I'll grab the bench bleeder kit from Advance Auto... When I did bench bleed it I used vacuum lines and I'm worried they didn't have a tight enough fit causing the issue.

Awesome anecdotal evidence/advice Nate thank you! Brake master cylinder is secret-level stuff I think you may be helping me get faster

Yes sir. MC is 15/16" 91 Civic EX without ABS.

 

well with a 15/16 MC... it should be fine with those calipers. it must be leaking or have air in it.

FYI I HATE the bleed kit from the parts stores. I ended up making old hard brake lines loop up and into the MC (they fit the MC the best) and added the hose from the kit to get deep into the MC. I bleed it in place... then swap one line at a time.

 

Great replies.
You didn't mention what pad you run, and the pads of your friends car.

 

Both Hawk DTC-60-- sorry..

 

Sounds like a brake booster problem to me.

 

Can someone explain to me this theory? Two replies now stating possible brake booster...

 

If you've got a bad booster that's bleeding off vacuum....well now you have no "booster" and feel a hard pedal.

 

when my booster was going bad it was "soft" without the car on, then would sort of come back but never fully failed. one big tell for me was that at idle, if you stepped on the brake the idle would jump (I don't know why)

 

It could just be a bad check valve on the booster.

 

http://www.norcalcrx.org/tyson/pics/braketest.jpg

and push rod adjustment since you have a non-native MC.
http://my.cardone.com/techdocs/PT%2053-0001.pdf

 

Awesome guys i will look into the BMC and the booster! Thank you!

 

Dil FWIW, I never bench bled my new 15/16ths master that I am using. I just used Russel power bleeders on all 4 calipers, bled them in sequence using a bleed bottle above the caliper and its been pretty good. If you use them put Teflon tape on the threads, mine would weep otherwise.

I get that idle surge too if I press on the brakes hard sitting still. =-S

 

There was a thread which i cant find... with Lo-Buck EF a few years ago.
Spencer had a similar issue and it carried on for a while.
Again this seems too familiar, and if i could find the thread we could compare it to yours.
But i think everyone else is on the right track here with your issue.

 

Ok so I did all of the tests.

master cylinder, booster, and check valve all appear to be fine.

I pulled the master anyway to bench bleed it as I suspect that could be a contributor since last time I did it I don’t think I used a seal tight connection for air at the two ports

when bench bleeding it the fluid was really dirty. Why would the master be dirty but the rest of my lines clean? Are the seals falling apart? Could this be causing my problem? What do I do?

 

yes, that was the only sign of issue i had and it was slight at that. better safe than sorry, throw a master in. i have a spare check valve on my bench somewhere you can swap in if you want too

 

Thanks Nate. I actually copped a new jawn off Rock Auto on Sunday and it should be here Thursday. Another 15/16th '91 Civic EX MC.

 

Alright, so I put the new master on and the brakes seem to feel better after a track day on a very short configuration at Pocono Raceway. I think some of what I'm feeling with the pedal getting a little big harder could be due partially to fluid expansion and maybe brake fade....I'll try bedding in my pads on my next set of DTC60s this time (I normally don't) and adding a master cylinder brace and some brake ducting.