Master cylinder and ABS Delete

[Kimo]

1989 Chevy OBS regular cab here and I am PERPLEXED. Here's my situation. I replaced the OEM Master Cylinder (MC) with a Little Shop Manufacturing MC in which the fluid reservoirs are opposite of OEM. i.e. Large is now in the front and the smaller one is now closest to the firewall. I also did an ABS delete according to this link.

Result....shorter brake pedal travel allowing for much improved response BUT, my stopping distance has noticeably increased as if the front discs are too small and there isn't enough fluid pressure to clamp down no matter how hard I get on the pedal. There just isn't enough stopping power. Additionally, even after I stomp on the brakes they wont lock up and despite all the great advice and how tos here in GMT, I haven't been able to determine if the reversed reservoirs are the cause of the diminished stopping power or I have something else going on.

Does anyone else run the LS Mfg MC on the Disc/drum setup or is the LS Mfg MC designed to only work correctly with the LS Mfg rear disc conversion?

SETUP: LS Mfg MC, OEM Proportioning Valve, OEM Front Disc, OEM rear Drum. ABS Module & Antilock Pressure/dump valve deleted

Front Disc Fluid path: Rear outlet of MC (small reservoir) --> 3/16 line to top front prop valve --> 3/16 line out bottom front of prop valve --> to T-block --> to front discs
Rear drums Fluid Path: Front outlet of MC (Large reservoir) --> 1/4 line to top rear Prop valve --> 1/4 line out of bottom rear of prop valve --> to rear drums

 

[df2x4]

or is the LS Mfg MC designed to only work correctly with the LS Mfg rear disc conversion?

I haven't looked at their site in a long time, but I seem to recall something about their MC being optimized for disc/disc setups.

 

[Pinger]

Result....shorter brake pedal travel allowing for much improved response BUT, my stopping distance has noticeably increased as if the front discs are too small and there isn't enough fluid pressure to clamp down no matter how hard I get on the pedal. There just isn't enough stopping power. Additionally, even after I stomp on the brakes they wont lock up

Caliper pistons are too small for the lost ability to generate pressure in the MC due to larger bore. Or, alternatively, the new MC's bore is too large for the caliper pistons you have. The latter, in reality.

From the web site: ''Larger 1.34" bore master cylinder to compensate for rear disc brake conversions and front caliper upgrades on 88-00 CK1500 trucks, where increased brake piston sizes''

That's my take on it. Others here can probably add more detail or dispute.
Isn't this the same issue as when fitting a GMT800 MC to a GMT400?

 

[Kimo]

I haven't looked at their site in a long time, but I seem to recall something about their MC being optimized for disc/disc setups.

df2x4, I bit the bullet and ordered the LS Mfg rear disc conversion kit. Hopefully this improves it. Worst case scenario, I put the OEM MC back on and stay with the Disc/Drum setup. I appreciate the insight.​

 

[Kimo]

Caliper pistons are too small for the lost ability to generate pressure in the MC due to larger bore. Or, alternatively, the new MC's bore is too large for the caliper pistons you have. The latter, in reality.

From the web site: ''Larger 1.34" bore master cylinder to compensate for rear disc brake conversions and front caliper upgrades on 88-00 CK1500 trucks, where increased brake piston sizes''

That's my take on it. Others here can probably add more detail or dispute.
Isn't this the same issue as when fitting a GMT800 MC to a GMT400?

Pinger,

Thank you for the info. I didn't take the larger bore into account. Reality; I failed to read thru and fully understand the rear disc conversion instructions before I purchased and installed. It goes without saying I probably should have. if just to know the science/math behind it. Shame on Me.

At this point I've decided to purchase the front caliper upgrade and rear disc conversion to go with my already installed LS Mfg MC. "If your gonna do it, do it right! " Thanks for the assist.

As far as the GMT800 to GMT400, I've done a bit of light reading since your post and it seems the larger bore falls in line with the LF Mfg MC science. And.....im guessing the larger pistons in the front calipers address the small reservoir in the LS Mfg MC otherwise, they wouldn't recommend buying all three as a kit.

Thanks again

 

[df2x4]

Wish I would've seen this earlier before you ordered more stuff but just FYI we have a couple members here who did the Little Shop disc conversion kit and were not super impressed with the stopping power afterwards. IMO it's more of a cosmetic and ease of maintenance upgrade than anything else.

The stock brakes can actually be pretty darn good even with ABS if you use some good pads and shoes and make sure the system is bled properly.

 

[thegawd]

I'm a firm believer in replacing brake parts by rpo code. some rpo's are better than others and if you get into swapping brake parts just because they fit, could actually impair the brakes due to mismatched master cylinders with mismatched brake cylinders or calipers. so if someone wants to upgrade the brakes you need all of the parts for that particular rpo code.

sometimes people report good brakes after swapping in a 2002 tahoe mc.... but it just so happens to be of the right size for their rpo code.... meaning they kind of just got lucky. I think JB3 might be the ****** brakes found on a lot of pickups and they could benefit from an upgrade. otherwise the others just need to be fixed.

IMO of course but.... but it's simple hydraulics.

Al

 

[Kimo]

Wish I would've seen this earlier before you ordered more stuff but just FYI we have a couple members here who did the Little Shop disc conversion kit and were not super impressed with the stopping power afterwards. IMO it's more of a cosmetic and ease of maintenance upgrade than anything else.

The stock brakes can actually be pretty darn good even with ABS if you use some good pads and shoes and make sure the system is bled properly.

df2x4,
No problem and I can always return the parts I haven't opened yet. However, there's just soo much information out there for these OBSs I end up heading down the "good idea ferry" path when it comes to improvement. It also doesn't help that this is my first build so, I don't have that trial and error experience under my belt. The good news is, these trucks are starting to skyrocket in price as more and more parts hit the market.

 

[Kimo]

I'm a firm believer in replacing brake parts by rpo code. some rpo's are better than others and if you get into swapping brake parts because just they fit, could actually impair the brakes due to mismatched master cylinders with mismatched brake cylinders or calipers. so if someone wants to upgrade the brakes you need all of the parts for that particular rpo code.

sometimes people report good brakes after swapping in a 2002 tahoe mc.... but it just so happens to be of the right size for their rpo code.... meaning they kind of just got lucky. I think JB3 might be the ****** brakes found on a lot of pickups and they could benefit from an upgrade. otherwise the others just need to be fixed.

IMO of course but.... but it's simple hydraulics.

Al

thegawd, Thanks for the info. Heck, I wasn't even aware of how the different bore sizes affected the pressure. This is all a learning experience for me. You folks have been super helpful.

 

[thegawd]

hey that is what were here for! and no worries. some guys on here are extremely knowledgeable on these trucks... I dont consider myself one of those guys but I'm always learning and dont mind sharing what I know. I almost did that Tahoe MC swap and have some of the parts.... well I read a few stories where the swap didnt work and I wanted to know more because I did not fully understand it. My Burb has JB6 and my sierra has JB5, both of which are more than adequate for these trucks if they work right.

I like to exercise my abs whenever I get the chance either on grass or snow. I think this exercise helps keep it in good working order. I live on 75 acres and can safely do that whenever I want...

take care eh! were about to get slammed by a storm. woooweeeeee! see ya in Kansas! ;-)

lol

Al