Okay, I have a 91 v6 pickup and I lost my brakes almost completly coming off an exit ramp (feaky feeling). I got it home and bled all the brakes and checked for leaks. There are no leaks but I did get a little air out of the lines. I took it for a ride around the block and noticed when I first step on the peddle it almost goes to the floor before stopping and does not stop very well. However, If I pump the brakes immediatly before trying to stop it feels like normal brakes and stops fine. If I wait a few seconds after pumping it and press the pedal again it goes to the floor. So, I replaced the master cylinder just now after bench bleeding it I took it for a ride and it's the same thing.... Any suggestions on what to do next? What could be effecting the pedal hieght like this when I pump it if it's not the master?

 

You are getting air in the system somewhere. Go over the lines with a fine tooth comb. Check the lspv as well.

 

I ran 4 quarts of brake fluid through the system... there is no more air.

 

With a new mc...and the pedal going to floor...then pumping up...air is getting in the system somewhere. Has to be. Or you just happened to get a new mc that was bad in the same way the old one was.

 

Doesn't matter how much you got or how much fluid you've ran thru it, IF it's leaking air into the system. Inspect the lines, and tighten the crap outta of the bleeder valves.

Note it doesn't have to be leaking fluid to be sucking air.

 

Okay thanx guys. The new MC is a rebuilt one from Autozone. I'll go check for leaks again and bleed the brakes again but it is entirely possible that I got a bad MC. Either way this is making me insane... Ive spent the last hours scratching my head.

 

Could it be a bad wheel cylinder or piston?

 

I had a pin hole leak for years, unbeknownst to me, along one of the lines you can't see well between the gas tank and the frame. Crud, salt, moisture collects there. The leaking fluid rotted my frame. Just a tip.

 

Well if you can't find any visible leaks from your brake lines pull your drums off and see if the wheel cylinders are leaking pry the rubber boot back and if its wet or if brake fluid shoots out at you then theres one leak you've nailed down. Did you bleed the master once you installed it?

 

Yeah, I'm aware of thst area collecting crap..Toyota messed up putting the lines next to that lip on the tank...

 

Yeah I bled it. A lot, just to make sure even the smallest bubbles were out, and even then I bled some more.

 

Maybe your rear shoes need adjustment? Most of the time when the rear arent adjusted properly they dont make contact with the drum causing the pedal to travel more so when you pump again they make contact cause of the extra fluid being pushed to the wheel cylinders.

 

Just an update- I found a slight leak in the rubber of the wheel cylinder so I replaced both of them... still the same problem. I made sure my rear shoes make contact and are adjusted properly....still the same problem. I am now going to exchange the new master I just put in and swap it with another brand new one and see if that fixes it.(maybe I got a defective one). If that doesn't fix the problem I am at a loss since I checked all the lines and they are not leaking and I'm not loosing fluid...

 

Okay, I replaced all the brake lines except for one short section and put in master cylinder after master cylinder. (I've now put in three master cylinders- I doubt they were all faulty) I replaced the rear brake shoes and properly adjusted them. I installed new wheel cylinders. I bench bled the master cylinders and bench bled them some more. I bled the whole system in proper order including the L.S.V.P. which I saved for last each time. I repeated this process over and over and over again. I've bled about 6 quarts of fluid through all of the bleeders just to be sure. No air comes out when I bleed it anymore. The master cylinder is not the problem, the wheel cylinders are not the problem, the rear shoes and their adjustments are not the problem, the lines are not the problem, they are not leaking. No air is coming out when I bleed the calipers so those can't be the problem either. Please help this is a nightmare!

 

stupid question, did you check your rotors to see if they had lateral runout or warped. that could cause your low brake pedal. at the first application of the brake pedal on warped rotors will be low, the is because the warped rotors push the caliper piston back.

 

I do have slightly warped rotors but always have. Could this have made me lose my brakes all of the sudden? They worked fine before....

 

Could it be the brake booster is faulty?

 

I don't believe so. Doesn't the brake booster just assist with stepping on the brakes by creating a vacuum? I think even if the brake booster wasn't there that I would still have brakes- it would just be harder to push the pedal down. Am I correct about this? Or if the booster is bad would my pedal go almost to the floor with hardly any braking and then once pumped again be normal??

 

your on the right track. basicly the booster uses engine vaccum to assist with braking. There are three basic stages

atmospheric port closed vaccum port open (when brake pedal is pushed down)
both ports closed when you hold on the brake
atmospheric port closed vaccum port closed in unaplied position (foot not on brake)

it is easier to see how it works in a picture. Although this is written at 11 pm so it might be a bit off lol.

i wouldn't machine rotors because it takes more metal off and is more likley to warp.
question- are your bleeders facing up right in your calipers? because if they are facing down, you will always have a low brake pedal

 

What do you mean facing up right or down? They stick straight out like they always have....I'm confused. All I Know is one day out of the blue my pedal hieght changed all of the sudden and went almost all the way to the floor and I hardly had any brakes... The same thing is still happening. However, If I pump the brakes I am able to stop normally. If I wait another 30 seconds and then press the peddle it goes almost to the floor and hardly any stopping power.