Brake issues, pedal goes to floor over time and front drivers locks

[BigBeech]

Hey,

I got an '81 Z28 that lately has had an odd brake issue. I noticed over time that the brake pedal at red lights would slowly push all the way to the floor and the car would start moving if i was on an incline. depressing and pressing it again would reset this and give about another 10 sec of stoppage while the pedal slowly went to the floor. on top of the when driving if i brake the car pulls left and if i brake harder the front left locks up quickly and the front right does not.

I chased this for a bit, bled all the brakes twice. then i replaced the proportioning valve thinking that it was malfunctioning and now I am at a bit of a loss. The master cylinder and brake booster are <2 years old and both are new units and are not a remanufactured.

When bleeding there were no leaks or low fluid output on either side so i am at a bit of a loss.
Not a brake expert here but when i was bleeding i left the master cylinder cap off and when pushing the brake fluid shot up a lot. My thinking is that when the pedal is pressed for a long period of time it is cycling fluid in a loop and not getting to the front right.

Let me know if you have any ideas what the cause of this could be, i am a bit tired of shooting parts...

Pic because who doesn't like pics?

Thanks

 

[Knuckle Dragger]

I'd say its the master cylinder bypassing. Any leak will do it but you usually see fluid. If the cylinder is bypassing it's all internal and shows no signs. 30+ years of doing this a slowly dropping pedal has always been a bad master.

 

[COPO]

Check for leaks on the floor from the MC. Most of them are made in China so don’t expect them to last for 30 yrs. if it’s leaking at the rear then it’s shot. Keep the cap on while pumping the pedal when doing a 2 man bleed starting with the furthest wheel away.

 

[BigBeech]

There are no leaks at all, i have been using the same volume of fluid for 2 years now and it started to happen a few months ago or so. I am inclined to think master but this would be my 3rd one in 2.5 years... it could be the master + some other issue that is causing premature failure on the master. I am willing to replace master, hardest part is bleeding would a clogged or throttled brake line cause excess wear on the master?

 

[Knuckle Dragger]

There are no leaks at all, i have been using the same volume of fluid for 2 years now and it started to happen a few months ago or so. I am inclined to think master but this would be my 3rd one in 2.5 years... it could be the master + some other issue that is causing premature failure on the master. I am willing to replace master, hardest part is bleeding would a clogged or throttled brake line cause excess wear on the master?

What brand master are you buying? There really isn't much that can take them out.

I reread your original post. I missed the locking up part before. You need to take a good look at the hoses, you may have one ballooning on you. If they are original I would change them regardless of how they look/feel.

 

[biker]

X2 on the master cyl. Pedal to the floor with no leak is a master cyl.

 

[72BIGBLOCK]

If you are sure there’s no leak on the master
Id narrow it to the rubber lines or the combination valve.

 

[BigBeech]

The first master cylinder i used was a doorman brand from autozone which went out quickly, i learned my lesson and then got a new one similar to this one, https://www.rockauto.com/en/moreinf...bRLj8rZJRVO9q3eeb+809HWf5KvmVM1bKwG8lyIq3Jw==

I will check the hoses and look at replacing them.

 

[Knuckle Dragger]

The first master cylinder i used was a doorman brand from autozone which went out quickly, i learned my lesson and then got a new one similar to this one, https://www.rockauto.com/en/moreinfo.php?pk=10961116&cc=1035335&jsn=13&_nck=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

I will check the hoses and look at replacing them.

Go with Delco Professional series parts when you can. They are a little more expensive but far better quality then the cheap stuff.

 

[mallard]

I've seen master cylinders fail without any leaks. Mine was working fine but had a tiny dribble at the back between the MC and the brake booster. Replaced with one from part source and it works fine. (and it even bench bled successfully)