slimbobaggins

Hey all,

Experienced brake pedal fade to the point that the pedal was pretty much to the floor, with no stopping power. I didn't see any obvious fluid leaks, and the master wasn't low on fluid, so I figured it was time to change my 28 year old master.

Bled the new master, installed, bled all 5 points (LR, RR, RF, LF, Prop Valve) until new fluid came out w/ no bubbles, brake pedal felt firm.

Started truck, and same thing, hold brake pedal down and it starts to fade. Engine RPM is changing with brake application, so I have to have some sort of vacuum leak at the booster. Could have been there before the master swap, I didn't notice.

Disconnected the booster vac line and plugged the port, and the pedal is back to firm, can hold strong pressure on the pedal and it never fades.

So... although it doesn't make sense to me how it would be possible, does anyone thing that my perhaps malfunctioning brake booster could be causing this pedal fade? I'm not sure what else it could be at this point?

the_supernerd

I am thinking that you have a small brake fluid leak somewhere that the added force of the brake booster makes noticeable. I had an '86 Camry that did the exact same thing and it turned out to be a bad replacement master cylinder from the parts store.

Try pulling back the boots on the wheel cylinders and calipers to see if there is fluid building up behind the boots. In my experience it seems that brake fluid starts to leak around the pistons and gets trapped in the boots before the leaks get real bad. There is always the chance you got a bad master cylinder, especially if it was remanufactured or really cheap, but I would rule out everything else first. It seems that the rear wheel cylinders tend to go out before the front calipers in most of the vehicles I have owned, and the leaks were never obviously dripping on the ground or anything like that.

slimbobaggins

Thanks for the response, I will pull the wheels and check.

It also occured to me that perhaps I have a line ballooning under booster pressure, that doesnt do it without the booster. I'm going to look for that as well.

the_supernerd

Good thinking. Any brake hoses that are cracked or have bulges should be replaced. Probably not a bad idea to replace all 3 of them if one is bad since they are cheap and easy to replace.

Daddyota

X2^

I had the same problem after swapping master cylinders doing the big brake upgrade. Turns out I had a cross threaded line coming out go the mc. That and I had trouble bleeding. Even after bench bleeding the hell out of it the pedal would still go to the floor. Finally after two bottles of brake fluid and a s-ton of bleeding the pedal firmed up and now the brakes are awesome.

slimbobaggins

Lines looked fine but Im gonna try to check under pressure today. Grabbed a booster this morning out of a '90 4runner

warmonger88

Check the vacuum supply line from the intake too.

offroadnutz

I have heard (but never experienced) that the LPV can go bad and cause an internal bypass which gives a fading pedal much like a bad master cyl. Could be something to look at.
You could also use hose pincers and pinch off the 3 rubber hoses. If it still does it then your problem is above the hoses, it it does not then your problem is below at the wheels.

Yota4Runner86

Heck my master cylinder has been bad for god knows how long, sometimes I hit the brakes and she stops on a dime, sometimes she has a full pedal, sometimes almost no pedal at all. I'll replace her when the brakes get really terrible haha

slimbobaggins

Meant to update, actually drove the truck, rather than just "test" the pedal in the garage, and it seemed fine when I drove it, no issues. Not sure if the brake fade after master swap was my imagination, or if I have an intermittent problem.

Still going to swap that master on when I get a chance, but going out of town so no time to do it right now.

the_supernerd

I have had that happen before. I think that a new master cylinder along with fresh fluid can sometimes make the pedal travel easier, so it feels like there is a problem until you drive it and realize all of the braking power is there. If it takes some force to get the pedal nearly to the floor and it hits a stopping point, that is probably ok as long as the brakes appear to be working normally while driving. Slowly sinking to the floor is bad.

If you bled the brakes with a vacuum pump, or something similar, you might try bleeding it with someone pushing the pedal while you open/close the bleeder valves. That method just seems to work better for me sometimes.