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1989 GMC C 1500 SLE 2wd 5.7 w/vaccum boost.
My brakes pedal is going all the way to the floor when I press it really hard. I can not find any external leaks. I assume i need to replace the master cyclinder. Rock auto shows two AC Delco that fit the truck. one is 28.6mm/40mm bore the other is 25.4mm/36mm bore . which one do I need? They are listed as if either will work........ see attachment
Look at your RPO code, you need to see which brake package your truck has, it will be a JB5, 6, 7. The larger bore would be for heavy duty brakes, if installed on a truck with standard brakes you would end up with touchy brakes, small bore on heavy duty brakes will give you a soft pedal.
And thanks for asking this, made me go looking and allowed me finally figure out my soft brake problem on my C2500, ends up I have the JB7 option, 8600 Lb GVW brakes which require a large bore master, PO had a totally wrong master on it when I bought it and I replaced it with a standard brake master and have been going crazy trying to figure out the issue with soft pedal, so your question solved my problem, new master will be here Monday!
Thanks for the information, but I am green as a gord! lol where do I find RPO codes? and what does the JB5, 6 or 7 tell me?
Should be a sticker in the glove box. If it’s not there, look at the sticker on the driver door jamb - it should also be listed on the registration. Being a 1500 it’s likely JB5 or 6 unless it’s a dually.
There’s actually 8 different possibilities for the JB codes (ending in 2-9), one of which will be the master cylinder code and one for the GVWR, so look for 2 of them. They’re listed alphabetically on the sticker.
If you respond with the codes, I’ll give you the definitions (easier than typing out each one).
In short, the RPO codes for brakes correspond to whether the brake system is vacuum-boosted or hydroboosted, and what the GVWR of the vehicle is. So, if you can't read your RPO sticker anymore, but the sticker in your doorjamb that has all the weight ratings is, you can figure it out from there. Here's the list of brake systems with a vacuum booster:
So with it being a 1500 I agree with Gumby22, that it is probably either a JB5 or JB6, The small bore master cylinder would be for the JB5 6400 Lb GVWR, the large bore for the JB6 7200 Lb GVWR.
J50. standard vac assist brakes, J55 in that year would be the heavy duty brake system, Looks like the JB code came later in life. So go with the small bore unit, 18M1743 and you should be all set. If you want to know what all your truck came with originally, google rpo codes, there are many listings that will give a description of each one of those codes in your pic.
Also check your rear drums. I had a long pedal and replaced MC & soft lines without change and eventually measured wear on drums and they were past limits. I had replaced shoes and adjusted but that wasn’t enough. Replaced drums and it shortened brake pedal travel. I’m not saying you don’t need a new MC, it sounds like you do. Just another thing which can cause long brake pedal travel.
1989 Gmc Master Cylinder question
