Funky brake bleed condition?? (1 Viewer)

[Sparky_Mark]

Hey all,
I must be missing something simple, but can't seem to track down the root cause of the problem...

'75 FJ40 with disk brakes -I think the first month of production. Original booster looks like it was for the drum brakes (the short one). New brake master cylinder, new lines, calipers (switched to disks in the rear), and fittings.

I tried to bleed the brake system and ran into a strange issue. The rear brakes were not getting any fluid. I thought it may be the proportioning valve, nope. I eventually remove the fitting to the master cylinder (I already removed the check valve). No fluid transfer from the resevior to the outlet! I could see the piston articulating inside, but no fluid. The front brakes blead fine.

I understand that my last course of action will be to remove the master cylinder and take it apart unless someone has any ideas. The cause must be trivial, brakes are not complicated and I have done them many times before.

The differences that could possibly be part of the problem are:
1) I bought the master cylinder for my '79, but used it on this one. I wouldn't think this would be a problem since the fronts blead fine. It is an OEM MS, not an aftermarket product.
2) The original booster is not the tandem type that was used for front disk brakes - the big tandem one like on my '79. I suspect that my vehicle was one of the first that they switched to disk brakes on and didn't think the booster needed an upgrade till later. I don't really think that the booster would have anything to do with the problem though.

Can I buy a clue? :dunno:
I don't mind feeling stupid if it gets me a solution to the problem.

 

[76FJ55]

the first thing that needs to be done with a master cylinder install it to check how far the rod protrudes out of the booster before bolting on the master. if this rod sticks out to far it will not let the piston return far enough to uncover the port to the reservior therefore not allowing fluid to fill the master cylinder. I don't know the dimensions off hand but if the master is for your 79 look in the manual for your 79 to get the dimensions for that master. I think they should be the same (both discfront ends) but not 100% sure. it also helps if you bench bleed masters befor you install them.

 

[Sparky_Mark]

I'll check, thanks.

 

[Shipwreck]

Did you bench bleed the master cylinder before installing it? If not, you can easily trap air in the rear chamber, not allowing fluid transfer to the rear brakes.

 

[megolfer]

I had the same problem with my brakes. I tied to bleed the back brakes but could get nothing to them. I tried just anbout everything. I thought it was the master so I got another new one. It did the same exact thing so I worked on it for many more hours. So finally I got another master it it worked great. I can almost bet that it is you master. Is yours new, and if it is where did you get it. I got 2 of mine from CCOT and they said that they had got a bad batch. I finally got a rebuilt one from a local auto parts store for $30. Problem fixed

 

[Erics75]

I would have to agree with Shipwreck, bench bleeding is a must, just be careful as sometimes the cups can be damaged by the compensating ports if you push the piston in to far. Getting a bad batch is very uncommon, also like 76fj55 said it's possible the pushrod length is a problem ( some are adjustable ) and sometimes the depth of the pushrod socket in the piston are diffrerent. One other thing to check for, make sure you have no kinks or dents in your steel lines.

 

[FJAfrica]

I had a very similar problem, and it turned out that the aftermarket seals I bought for the MC were ever so slightly thicker than the original. The result was that when the piston was fully back, the lip of the seal just covered the hole to the reservior, preventing the fluid from entering the mc. &nbsprove me batty trying to figure it out!

 

[Sparky_Mark]

Guys,
Thanks for the suggestions. I'm going to go at it after work tonight. Unfortunately, I dry fit everything for packaging a while ago and got lazy a few nights ago. I did not remove the MS to bench bleed it .
The MS (OEM) is new from a reputable source locally. It has sat on a shelf for a few years though. I have removed the steel line from the MS and have isolated the problem to the unit (or the rod length on the booster). While scratching my head on it the first time, I remember looking at the output port and the opening in the bottom of the resevior - no obvious blockages, but I'll check again now that I know what I'm looking for.

Thanks very much for the suggestions, we'll see what happens tonight.