Need Help With Brakes

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45Kevin

45Kevin

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Some background:

I put (well actually Crushers did) the front axle, complete with the disc brakes, from a 1983 FJ43 on my truck about 6 years ago. The master cylinder was the stock one for the original drum front brakes. Ever since, I have had to give one pump to bring the brakes up to good working pressure. I figured that was due to the original drum M/C.

Now recently, I have noticed that the brake pedal would slowly sink down to the floor when I was waiting at a light or on a hill. All the opinions I got suggested a bad M/C.

Last week I bought a new M/C for a 1990 FJ80. I did this because of the larger bore of the 80 M/C. I had looked at M/C for a 1978 FJ40, but the bore was much smaller. With the huge drums in the rear I figured the larger bore would be the way to go.

I have installed the 80 M/C and now the pedal only seems to go down about half way and feels like it bottoms out. I backed off the push rod (it only had about 1/8" to go) with no noticable change to the pedal.

What the hell is wrong? :mad: Many people suggested the 80 M/C as a direct bolt-on replacement.

I'm sure a few of you have converted to discs and gotten good results.
 
45Kevin said:
Ever since, I have had to give one pump to bring the brakes up to good working pressure.
Click to expand...

This to me says mis-adjusted rear brakes.
 
45Kevin said:
I have installed the 80 M/C and now the pedal only seems to go down about half way and feels like it bottoms out. I backed off the push rod (it only had about 1/8" to go) with no noticable change to the pedal.
Click to expand...

Do your brakes work correctly? They are stopping the truck?

If you install a MC with a larger bore you should end up with less pedal travel. (assuming the brakes are still the same) You're displacing a lot more fluid which will engage the brakes faster.
 
RufusTheDufus said:
Do your brakes work correctly? They are stopping the truck?

If you install a MC with a larger bore you should end up with less pedal travel. (assuming the brakes are still the same) You're displacing a lot more fluid which will engage the brakes faster.
Click to expand...

NO, the brakes are not very strong. They will stop the truck but will not lock up the front wheels in the gravel. It is like you put a block of wood behind the brake pedal that only lets you push the pedal half way down.

The 45's have a bigger bore M/C than a 40. The 80's also have a bigger bore that a 40.
 
I think the MC in my HJ45 and BJ42 are the same. They are both Australian market with dual-cylinder drums all around.

Did you try to back off the adjustment on the rear drums to see if it gives you some more pedal travel?

Does the 80 series come with drums in the rear or discs? Just curious. I have no idea what's in them.
 
RufusTheDufus said:
Did you try to back off the adjustment on the rear drums to see if it gives you some more pedal travel?
Click to expand...

Why would you do that? Drums need to be adjusted correctly first. If they are correctly adjusted then the issues are elsewhere. Adjusting the rears so they will take more fluid doesn't fix the root problem.
 
cruiser_guy said:
Why would you do that? Drums need to be adjusted correctly first. If they are correctly adjusted then the issues are elsewhere. Adjusting the rears so they will take more fluid doesn't fix the root problem.
Click to expand...

I agree. This was a question to help diagnose the root problem not fix it. i.e. are the rear drums locking up and preventing the fronts from doing any useful work?

Lots of folks have to use proportioning valves to get proper front/rear bias when changing from non-stock brake components.
 
Some info that might help. (I remember reading this thread when I did a front disc conversion on my 67 Mustang.)

To figure how much pressure your master cylinder is putting out:
C = pedal ratio
D = pounds of pressure apply by your foot
E = area of you master cylinder
F = pounds of pressure out of the master cylinder
C X D /(divided by) E = F

Example: If you have a 1" master cylinder the area equals 1/2" x 1/2" x 3.14 = 0.785 Square Inches. So, 100 pounds (of applied foot pressure) X 6 (pedal ratio) divided by 0.785 = 764 pounds of pressure.
If you have a 1-1/8" master cylinder, 100 psi X 6 (pedal ratio) divided by 0.9935 = 604 pounds of pressure.

Here is some info on master cylinder with "constant" of 6 to 1 pedal ratio and 100 psi being applied.
3/4" master cylinder = 1359 psi
13/16" master cylinder = 1158 psi
7/8" master cylinder = 998 psi
15/16" master cylinder = 870 psi
1" master cylinder = 764 psi
1-1/8" master cylinder = 603 psi
Click to expand...
 
Well, before I put the new M/C the front and rear would lock up if I wanted them to. The issues were, the additional pump of the pedal to get the brakes to work, and more recently the way the pedal slowly sank to the floor when I was holding the brakes on.

On a different note...how do I get the rear drums off. I took the big phillips head screw off but the drum seems to be on there tight. I whacked it few times with the hammer, but it feels as if there is some other mechanism holding it in place. I can move it outward by about 1/8 - 1/4" if I place a big straight screwdiver inbetween the drum and the backing plate and twist.

There are two holes that look as though I could screw in a screw to push the drum away from what ever it is sitting against. Like the two screw holes in my axle flange to get the axle out.
 
In regard to your first problem, before the master, that might still be an issue, it seems to me that you may still have air in the wheel cylinders in the rear, or problems with your flex lines.

I finally used a brake bleeder that pushed fluid back up the lines from the wheel cylinders to get it all cleared out.

On the flex line, when is the last time they were replaced?
 
Rufus, I'm having a tough time wrapping my head around the pressures you posted up. It would appear that the larger the master bore the lower the pressure at the brakes but it doesn't make sense then to use a larger bore master as the pressure applied at the brakes would be lower thus making harder to lock them up? Either the chart above is wrong of the idea of a larger bore master being better is wrong?
 
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Thanks for all the insight guys, but what I really want to know is why the pedal now seems to bottom out about 1/2 into its travel. It has not built up sufficient pressure at this time to adequetly push out the pads to lock the wheels.

I did bench bleed the master and then I bleed the wheel cylinders enough to get the new (clear) fluid running out the bleeder screw. I am pretty certain that no air is trapped in the system.
 
Regarding the pressure.
It is the relationship between the M/C bore and the W/C (or caliper pistons) that is important. If you put in a smaller bore m/c then you will either need a longer stroke, or get less presuure at the caliper / w/c. That's why I decided to go with the 80 m/c. It does have a shorter stroke by the loos of it.. That may explaint the bottoming out 1/2 way through the pedal travel.
 
cruiser_guy said:
Either the chart above is wrong of the idea of a larger bore master being better is wrong?
Click to expand...

The chart above is not wrong. The idea of a larger bore is not wrong either. Sticking a larger bore into a brake system not designed for it is wrong. A larger MC bore requires a larger vacuum diaphragm or a different pedal ratio to generate the same PSI as a smaller bore.

A larger bore will move more fluid in less pedal travel causing the pedal to bottom out sooner, albeit at a much lower pressure. Larger MC bore size does not equal 'better'.
 
I don't know anything about the 80 series. this post in the 80 forum might help. Near the end of the post it says this:
One more thing. There is adjustment on the master cylinder between the pedal and master cylinder that has be adjusted with the master cylinder off the vehicle. This usually affects pedal travel, but it's possible that yours is not adjusted properly and therefore you have maybe 80-90% of your brake power, but not 100%.
Click to expand...
 
RufusTheDufus said:
A larger bore will move more fluid in less pedal travel causing the pedal to bottom out sooner, albeit at a much lower pressure.
Click to expand...

Isn't this pretty much what the OP is describing?
 
I think the bores are the same. That is why I (and others) used the 80 series m/c. I had never considered the stroke, which I assume is related to the pedal ratio. I am assuming that the pedal ratio is between the foot pad on the pedal and the point at which the push rod is attached to the arm of the pedal.

So... maybe the stroke of the 80 series m/c is much less that the stroke of the original (the body is several inches shorter) and because of the shorter stroke, it would take less air in the sytem to cause a problem. :hhmm:

I'll bleed the rear brakes next.
 
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