So I was having this problem where out of nowhere the pedal would go to the floor. Then id pump it back up quickly and it would come right back. I figured it was the master I swapped the master and attempted to bleed the brakes when I bleed the fronts which are the ls1 brakes from bigbrakeupgrade.com the pedal drops a ton. When I go to open the rear there is very low pressure and the pedal hardly travels down. I kept cracking the lines all the way to the master and still cant get the pedal to drop when I opened it. So I figured I had a defective new master and swapped it out again. Same issue. I did notice if I pumped the e brake a few times, oddly id get a little more pressure in the back. So I thought that was fine so I road tested it and was fine at first but then the "brake light" comes on and off and the pedal is just mush. Not going to the floor like before but just a real crappy pedal. Thoughts?

 

Rubber brake hose at the rear axle pumpkin

 

even if i crack open the line in fronnt of it i dont get much pedal travel or pressure

 

I have never seen it happen, but I am going to suggest you are having issues with your proportioning valve. If the internal piston is not centered, it will block off (or partially block off) flow to one circuit. The brake light on the dash is illuminated by this shifting to one side or the other.

The other idea is, you have crud in a brake line that is choking the system.

 

I’m having a similar issue on my car but I haven’t got around to diagnosing yet since the cars apart for ls swap/paint. I switched to 4th gen rear and replaced all hoses but couldn’t ever get any real pressure to the rear. Im gonna look into it when I get the car back together. I was assuming I had a master cylinder issue.

The only thing I can suggest is to do what I plan to do first and that’s just to keep bleeding them and maybe you’ll get somewhere.

 

yeah i bought the attachment for my pressure bleeder for that master. im gonna try to pressure bleed it first.

 

Hope you are not bleeding it installed.

 

na i bench bled the master first, gonna use the power bleeder on the system once its installed.

 

*IF* there is still air in there, I don't think that will get it out installed. The master is installed with too much angle to make sure you would get all the air pockets out - none of the outlets or inlets are located remotely near the top of the bore. Doesn't matter the method - it's about how it sits in the car. If it works, please let me know.

Bench bleed level, put plugs in the outlet ports, push on the cylinder and measure travel. That will be a good indication all the air is out. A new OEM MC had a spec of 1/8" free play travel - by the Haynes manual (take that for what it's worth ).

 

i dont think my problem i in the master

 

Part of the forum experience is recognizing when someone doesn't know what they're talking about, and taking their advice with a huge grain of salt.

 

Are you saying the brake pedal won’t go all the way down?

 

when i open the bleeders there isnt much pressure and the pedal down go down much if at all but only the rears. even if i crack it in front of the center brake hose

 

Well that’s definitely different than my issue. My pedal feels fine I’m just not getting full pressure to the rear brakes. It doesn’t sound like bleeding is your answer

 

Get someone to pump the brakes and hold pressure, then crack the fitting loose on the out side on the line to the rear brakes. Is there flow, and if so how much? Just a trickle or a good stream?

If you have a good stream it means there's an issue between the prop valve the the rear calipers. If no flow or weak flow, you've got a problem between the prop and the master.

Sounds like there's a bad prop valve...