Proportioning valve or different Master Cylinder for disc brake upgrade issue?

[Jacob Bridges]

1974 US FJ40 with F155

Hello, several years ago I installed Man-a-Fre's Wilwood front disc brake kit and GM style rear disc brake kit. At the same time I swapped the master cylinder to a Dorman NM55034 (1993 FJ80 Master cylinder) as I believe I found recommended here. I'm still using the original brake booster. It always takes two pumps of the brakes to stop/slow. Second pump is firm, no slow pedal drop or anything. I've probably put 100 miles on the FJ40 since installing the brakes so it's been a very low priority. I'd like to get them working correctly. Brakes have been bled multiple times, I don't believe that is the issue.

It was suggested to me that the system may not be pushing enough fluid for the calipers I have and to install a proportioning valve on the rear circuit. I also saw on CityRacer's site, he suggests a 95-97 M/C from a FJZ80 for 4 wheel disc brake applications, and suggests the 93-94 FJ80 I have for front disc/rear drum brake setups.

So finally to my question- would you first try:

1. Add a proportioning valve to the rear circuit
2. Change the M/C to an OEM FJZ80 unit from CityRacer
3. Something else entirely

I'd like to get my FJ40 roadworthy/family worthy and this is one of the non-negotiable I'd like to get sorted. Thanks for your help!

 

[pb4ugo]

What's the bore diameter of your current master?

 

[Jacob Bridges]

What's the bore diameter of your current master?

I believe it is a 1" bore (assuming the Dorman one is the same spec as the OEM FJ80 it is patterned after).

 

[pb4ugo]

I think you have a volume issue. I dont think a proportioning valve will help. I ran into a similar situation decades ago. I was running a fj40 disc brake frt end and gm disc discs in the rear with a stk fj40 7/8 diameter master for the 76 40. Everything worked fine. I later installed a gm 3/4 ton truck disc frt end. It had larger calipers and it took 1 pump to get a good pedal. I installed a fj40 1" diameter master for a 72 40 and it works perfectly. I'm not sure what you would need. Maybe a larger bore or a longer stroke or stk calipers. A larger bore is going to create more pedal resistance, but you do have power brakes.

 

[pb4ugo]

Have you checked or adjusted your pedal height per the FSM? You may find some more stroke.

 

[nabbasi]

I just did some research on this recently. The MC you have was for front disc/ rear drum and the later one was for disc / disc w/ abs. So I’d assume the residual valves would be different. It does sound like a volume issue though.

If it were me, I’d start with making sure that your booster depth is correct. If that didn’t work, then I’d add the adjustable proportioning valve. If that didn’t work, then swap the master cylinder, and leave the adjustable proportioning valve.

 

[Jacob Bridges]

Thank you for all of the feedback. I think I'll start with the prop valve first, followed by the different M/C if that doesn't work. I may change my mind and do the M/C first since I won't have to source a brake flaring tool to install that...

Either way I'll report back with progress.

Also, I adjusted the brake booster rod a couple of years ago so that shouldn't be the issue. I have not set the pedal height per the FSM so I'll give that a look as well.

 

[Jacob Bridges]

Have you checked or adjusted your pedal height per the FSM? You may find some more stroke.

I measured my pedal height and found it sits at 7 5/8”. So nearly 2” short based on another thread here. Unfortunately I have a GM steering column installed by the PO and I can’t adjust it out any further.

Great recommendation to check the “basics” before buying new parts…

 

[peesalot]

Hi Jacob, I am in Windsor, my 1970 dual circuit MC works 4 wheel disc fine. Do you have the metric GM calipers or the 3/4 ton bolt on ones ? I have the /arger GM calipers in front & the metric ones in rear, no proportioning valve, a boosted 1970 dual circuit MC and they work well. Which calipers are you using front & rear ? May be a bad MC.

 

[pb4ugo]

I'm not familiar with using a GM column. How is it interfering with the brake pedal. I think you need that additional 2" or a much larger diameter master, which will increase the resistance in the pedal.