Brake Problems- HELP Please!

[Beluga]

After converting to disc brakes front and rear on my 73 FJ40 and running the stock master and booster for years, I decided an upgrade was in order. So I purchased a dual diaphram 8" booster and a 1 1/8" Wilwood disk brake master (without check valves). I bled the master back into itself until all bubbles were gone, before connecting up the brake lines. I made sure the front lines go to the larger reservoir on the master. I Bled each caliper thoroughly about five times now. Haven't gotten any bubbles in last several tries, so I'm confident there is no more air in the lines.

When I press the pedal down it goes fairly easily about 1/2 way to the floor then stops solid (as if a block of wood was between the floorboard and the pedal). Braking action is far from adequate. Pumping the pedal won't help. At that point no further braking action will happen no matter how hard the pedal is pushed. If I open one of the front bleeders at that point the pedal will continue nearly to the floor.

At this point I am stymied. The only thing I may look into tomorrow is the push rod length, only because that's all I can think of left to check.

The front calipers and rotors are off an FJ60 I believe, and the rears are the typical chevy conversion.

Any ideas on how to troubleshoot this?

 

[igotta40]

Had the same symptoms you describe, and when I set the booster push rod length that fixed it. BTW mine's a drum brake with booster (stock 1974). Had a helluva time locating the adjustment tool until I made my own... worked great.

 

[Beluga]

I messsed around with the length of the push rod for a couple hours today to no avail. Any other ideas? Anyone?

 

[61n72cruisers]

check to make sure that the calipers are contacting the disc when the peddle is fully depressed

 

[Beluga]

I jacked it up so al 4 wheels are off the ground and had my wife push the pedal down while in neutral. I was not able to rotate any of the tires by hand so the pads are contacting the disks.

 

[Fast Eddy]

Take it all apart again rolleyes, and measure the travel of the master cylinder piston. Then measure the stocker and compare them.

It actually sounds like the master is only engaging the rear brake circuit, on only applying power to the rear circuit. Is it driveable? Take it out to a gravel lot and see what happens when you lock it up.

Did you put a proportioning valve on the rear circuit?

Too late now, but you should've gotten an fzj80 master. Works great.

 

[Beluga]

Took it out to a gravel lot. Hit the brakes hard at 5 mph. barely locks rears, won't lock fronts.

With master removed, pedal will travel easily all the way to the floor. Push rod extends quite adequately out of new booster(didn't measure it earlier today when I worked on it). As I say, when a bleeder is opened on the front, the pedal will be allowed to go past the previously "rock hard" point, and continue 4-6 inches further to the floor.

The front calipers were purchased used for the conversion, and may have been in the boneyard a while. Maybe the pistons are stuck, there's 2 on each caliper. To get the caliper off I have to remove the fluid line. If I remove the disk, then reattach it without the disk I can test it to tell if a piston is stuck.

I'm assuming the brand new booster is working properly. I'm running an unmodified 5.3 vortec so Vacuum shouldn't be a problem.

Here's some info I found on the net.

[SIZE=-1]Rock Hard Pedal [/SIZE]
[SIZE=-1]•Insufficient Vacuum [/SIZE][SIZE=-1]•Check vacuum pressure See Test Power Brake Booster[/SIZE]
[SIZE=-1]•Blocked Vacuum Check Valve[/SIZE][SIZE=-1]•See Test Power Brake Booster[/SIZE]
[SIZE=-1]•Bad power booster[/SIZE][SIZE=-1]•See Test Power Brake Booster[/SIZE]
[SIZE=-1]•Frozen Wheel Cylinder[/SIZE]
[SIZE=-1]•Replace caliper[/SIZE]
[SIZE=-1]•Binding Pedal linkage[/SIZE]
[SIZE=-1]•Inspect pedal assembly[/SIZE]

The master cylinder diagram shows two spools inside and a couple springs. I am thinking that has something to do with balancing the pressure or volume to front and back calipers. The directions say to check the volume of the cylinders and connect the line that goes to the greatest volume cylinders to the front (larger reservoir). The rear pistons measure 2.10 inches in diameter. I haven't measured the fronts. No I don't have a proportioning valve in the system.

 

[65swb45]

Disconnect the booster and see if it works any differently. My hunch is that the problem is in the booster.

 

[Beluga]

Success at last!

Today I swapped out the 8" booster for a 9" unit and it made all the difference in the world! Both units were new off the shelf, but I have to wonder if there wasn't something wrong with the first one (8"). I followed the testing procedures I found on the net but wasn't able to find anything wrong with it. My stock booster was 8" but single diaphram (ran it for years with discs on all wheels and it was acceptable) so I thought the dual diaphram would be an improvement even though it was the same size. I didn't have much room for the 9" and will have to relocate the ECM slightly. Even though there is just one inch difference in diameter the 9' is visibly much larger. I'm one happy camper! Now I think I'll get a proportioning valve. Any advice on these?