75 Power brake booster

[Pibe_tuerca]

Hello i bought a couple of month ago a 75 custom deluxe. 6L 250 with TH350, she didnt run from 2001. one day of work and now she is running, before i change all the fluids before and after the first run. Before do a test drive i check the brakes and the master cylinder is dead and the boosteer is blowing inside doing a nice "swooshhh" so i suppouse that the brake fluid distroy the inner diaphragm.
Time to buy spares, here come the question, how i know the type of booster that i have? JB3, JB5? please help. Attached pictures of the booster and brakes.

PD: the front disc 11" and rear drums 2"

Thanks!

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[1987 GMC Jimmy]

Go ahead and check your glovebox for an RPO sticker. The JB* brake code should be there.

 

[Pibe_tuerca]

Thanks but the sticker of my truck it's partially destroyed as you can see in the picture. that's because i am asking how can i know wich type of booster and brakes i have.

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[75gmck25]

My '75 GMC has J55 code for the heavy duty power brakes, and its on the left half of the sticker. All the HD options on my Camper Special seem to be listed on the left side (brakes, battery, stabilizer bar, etc.).

I installed a new brake booster and master cylinder about 6 months ago. However, the 3/4 ton K25 does not have choices of JB3 or JB5 for boosters so NAPA had no difficulty determining what I needed. It all matched up and worked fine.

I looked through the GM shop manual and it does not explain how to differentiate between JB3 and JB5 boosters. However, this link may be helpful to determine what you has from the factory. http://67-72chevytrucks.com/vboard/showthread.php?t=209390

Bruce