B4Runner

My old master cylinder decided to kick the bucket. No problem, got a new (remanned) one. The new one, once installed, decided to be defective. I came to find that it only pushed adequate fluid to one set of lines. Luckily, I bought it with a warranty and got another new one.

This new master cylinder builds up pressure to my front breaks and will eventually bring the truck to stop. I have to have someone hokld the pedal as I let some fluid out of the front line on the MC.

But this really doesn't make sense to me, I don't know what's going on, and I can't find any posts with the similar problem.

I made sure I properly bench bled this MC. I got it installed and it has worked properly and efficiently for the past 4-5 days. But as I'm driving into town yesterday, all of a sudden, it decides to build up the pressure and make me pull over on the side of the highway. Now, when this happens, I let the fluid out, and I can only drive for a little amount of time (utilizing the brakes as necessary) until I have to let fluid out from the front line again.

I really don't know what's wrong or why it's happening. Any advice would be great! Thanks!

highonpottery

iTrader: (1)

sounds like an odd issue you've got going on there. definitely not safe to be driving around with iffy brakes, so let's get you fixed!

may seem dumb, but besides bench bleeding the MC, did you do a full pressure bleed of the entire brake system? if not, I would definitely start there and make sure that you've got HEALTHY squirts of FRESH fluid coming out the bleeder valves. it could be there is some sort of blockage in the lines and this way you can eliminate blockage and/or old dirty fluid from the diagnosis. remember to do it in proper sequence, furthest from MC to closest (R-R, R-L, F-R, F-L), and remember to keep topping off the MC fluid, otherwise you won't be sure to have all the air out.

now, are you having to let fluid out of the system because the brakes are locking up on their own? if so, where are you bleeding from? each time you crack the lines to let pressure out you're potentially introducing air into the system, which will cause malfunction. when is the last time you worked on the brakes themselves? there could potentially be an issue with your caliper pistons, but since i'm assuming everything was all good before the MC then it's probably not the issue.

give us some more details on what's going on with the brakes and what you've touched on the system so we can get you back on the road, er...trails

the_supernerd

Make sure the push rod from the brake booster is adjusted properly. If it is too tight, it will cause the brakes to drag, which sounds like the problem you are having.

Personally, I like to get new brake parts. I have had bad remanufactured parts that do not work like they are suppose to.

Gordo

I had that happen in a datsun 280Z i used to own. Your master cylinder is bad, It is not releasing pressure .Each time you use the brakes it should increase pressure (push pads to drum or disk). When you let up it should release that pressure back to the master cylinder. Your’s is not doing that. When this happen to me it was perplexing. Finally, I replace the master cylinder that corrected the problem

justin_4runner

yeah u got another bad mc... its not releasing the pressure. and u might want to just spend the extra money and get a new.

highonpottery

iTrader: (1)

Might as well upgrade it to the 1" bore vs the 3/4" if you're replacing it again.

Zelephant

Don't do that unless you upgrade to a double diaphram booster. IIRC, the 1" MC needs significantly more force to push in, running it with a regular booster would be like having manual brakes.

I swapped both at the same time, easy and worthwhile mod.

fjmudder1

Get a new master cylinder. I use the stock size and have excellent braking. I also have 31x10.5 tires.