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Soft Brakes after Disc Conversion with a 383 Stroker

PJ Walters

PJ Walters

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Jul 25, 2021
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Boiling Springs PA
Good Morning Folks, I try not to burden ya'll too much with my questions, but I got a real pisser here that I cannot figure out. I searched all the old threads and there is A LOT of info, but I cannot seem to find anyone with my issue. Hopefully you all can help me out. I have a very soft brake pedal with crappy stopping power and before someone screams "you got air in your lines", I can assure you that after we pressure bled the system there is no air in the brake lines. The first issue is that I could lock up the rear brakes so I thought, maybe the brake lines got switched at the PV? I googled diagrams and mine was plumbed right. However, I did notice that on some MCs the 1/4 brake line is closer to the booster and the 3/16 line is in the front. Well mine was reversed and just bc I am throwing noodles at the fridge door, I changed them around. It did provide a little more stopping power and my rear brakes really lock up now (to the point that it tosses the truck sideways). However, I cannot get a definite answer if this COULD be the issue. Do some MC have the lines switched around? By all accounts, I should have a stiff brake pedal due to no vacuum, but that is just not the case. Again, we hooked up the MC reservoir to a pressurized bleeding system at my buddies garage and there is no air in the lines. All my brake lines are brand new and the rubber lines to the calipers are not expanding and giving me this issue as I clamped them off. Lastly, this is the third truck I did the disc brake conversion to. I have never had this problem and the only two things different is I have never changed the PV to disc/disc in the past and I never had a 383 stroker. This may be a dumb question, but are some disc kits made to go with a disc/drum PV? I should have jest got a hydro boost...

89 Blazer 383 stroker motor

Brand new brake booster
Brand new master cylinder (Napa M2424 1 1/8 inch bore) (I changed this twice btw)
Brand new brake lines all around
Brand new proportioning valve (switched from Disc/Drum to Disc/Disc)
Disc brakes rear (I cannot remember the kit, but they are El Dorado calipers)
 
I am not a total expert on rear disc brake conversion, but you could check to see how much vacuum the booster is getting. I have also seen people on this web site say they had to buy an expensive adjustable proportioning valve, in order to get their rear disc brake conversion to work right.
 
Engine displacement is irrelevant. If you have a giant cam so the engine doesn't make much vacuum, that can make a difference, but you said the problem is a soft pedal.

Cruise through the sticky hydroboost thread, as it tells about the different MC over the years and which port is for front and rear. Also search on the PV bleeding procedure. Having that open has caused many headaches. Here's what I can think of for soft pedal:
  1. The rear calipers need way more fluid than the old wheel cylinders and this MC isn't getting there.
  2. There's air. I know you're sure there's not, but still search those threads. I always wanted to try drawing clean fluid into a caliper and sucking it out of the reservoir, but I'm pretty sure the bleeder screws would leak air into the system.
  3. There needs to be "residual pressure" valve on the MC (or somewhere) to keep from returning too much fluid when the pedal comes up.
Eldorado calipers are pretty small, so they're probably within the ballpark of matching the stock fronts, but if you catch the rear locking up first on heavy braking, you may still need an inline PV for the back.
 
Make sure the bleeder screws are at the top of the caliper and not at the bottom where air bubbles would get trapped.
 
I would suspect master cyl or still trapped air….I know you replaced the master but I got 3 bad ones last year before I got a good one….
 
I would switch the lines back. The front brakes do most of the braking.
If your pedal starts out firm-ish, then sinks, your master cylinder is by passing internally.
Do you know what master the kit provided?
What year is this truck? Did it have rear wheel abs ?
 
rampage said:
Make sure the bleeder screws are at the top of the caliper and not at the bottom where air bubbles would get trapped.
Click to expand...
Yep good advice, the rear calipers on mine are taken off the rotor and spacers put in during bleeding so the bleeder screw are up high during the process. Once done with that caliper it is put back on the rotor.
 
Wes Harden said:
I would switch the lines back. The front brakes do most of the braking.
If your pedal starts out firm-ish, then sinks, your master cylinder is by passing internally.
Do you know what master the kit provided?
What year is this truck? Did it have rear wheel abs ?
Click to expand...
I am going to switch them back. My pedal doesn't start out firm, its soft from start to finish. The kit did not require a change in MC. It is a 89 with no ABS. I am going to purchase a larger bore for my MC and install a 2 lbs PV in my brake line. That seems to be the resounding answer.
 
So a disc drum MC has a check valve in the rear drum circuit. Normally you either remove the check valve , switch to a disc/disc master. Most rear brake upgrade kits come with a corvette spec master cylinder. For our trucks I would look for a 90's vintage P30. these were lower gvwr and had disc disc, or possibly a gmt800 2500hd application, you might need to re-flare the MC brake lines changing to the correct flare nuts
 
A disk/drum master is going to have a different volume of fluid for ea circuit. Disk needs much more fluid than drum.

When I did my 4wh disk swap I changed out to an early 2000's 2500 master. Still ran on vacuum, but had plenty of volume for the all of the disks.

80 Jimmy - The Monkey Paw (Time to finish interior)

Started on the B pillar that someone had pushed in some steel wool and bondo’d over.. Cut out the steel in there and replaced w/ 16ga. Came out ok. There were some factory spot welds, so decided to try rosette welds there, and made the holes I drilled in the outer panel a bit large. May go...
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Wes Harden said:
So a disc drum MC has a check valve in the rear drum circuit. Normally you either remove the check valve , switch to a disc/disc master. Most rear brake upgrade kits come with a corvette spec master cylinder. For our trucks I would look for a 90's vintage P30. these were lower gvwr and had disc disc, or possibly a gmt800 2500hd application, you might need to re-flare the MC brake lines changing to the correct flare nuts
Click to expand...
Wes, Thanks for this info... My current MC bore is a 1 1/8" bore disc/drum (Napa M2424). A P30 is a 1 1/3". Instead of a PV to the rear, this looks like an easier fix. I am learning something new everyday! HA
 
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