Rear Disc Brake Upgrade Update

I know quite of few forum members thought that is was a waste of time to convert to rear disc brakes.

I reported what I found with a much improved pedal feel. Nos so hard of pedal etc.

I have driven this for several months and contemplated to going to bigger rotors and 4 piston etc because I thought it should be better than what I was feeling but have been too busy to do anything about it.

However I knew I did not do anything with the disc/drum master cyl and I knew that was not right.

So I decided to check the caliper pressures knowing that the rear and fron should be about 1000-1100PSI

Mind you that this is a K3500 with a hydroboost.

I purchased a pressure gauge and checked it and found the rear disc was only 475psi and the front was only 675 psi

I was shocked and happy at the same time.

I installed a new disc/ disc master cyl with a 1 1/8 bore which should be correct for the conversion with no residual or prop valve.

I installed the new MC and the readings are now 1050 in the rear and 1100 psi in the front.

I took it for a drive and was delighted as it feels like a newer truck with a very nice normal feeling pedal and unreal stopping power from what I was experiencing.

No more hard pedal feeling and no more dull feeling pedal. It has the feel you should expect a truck to have even though it was an older truck. To me 4 wheel disc brakes should feel the same on any year truck because the brakes do not know a thing..

It pays to check things more closely and the pressure gauge told the story.

Converting to Disc Brakes all the way around with a new correct MC with the correct bore size changed this truck stopping power 100%

I let a buddy of mine drive it who has a 2015 GM 3/4 ton and he says it feels and stops as good as his truck

Best thing I have done to the truck as far as safety is concerned.

Go ahead naysayers and attack!!...but for those who want to do this conversion I would be happy to share info

Which master cylinder did you purchase?

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Cokeman95 said:

Which master cylinder did you purchase?

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Click to expand...

Exactly!

Master Cyl is a commonly stocked item Raybestos # MC 390363

This is an aluminum master cyl...a direct bolt on with the correct bore for 4 wheel discs
I did not use a prop valve but was ready to in case the rear brakes locked up first....tried it on gravel.

This will give you the correct pressure on all 4 corners. I think it works so well is the GMT 400 trucks are much lighter than the new trucks and probably dont need as much brake.

I used the JB6 or 7 rear calipers which is a single piston

As it stands now I will leave the brakes alone as they seem to perform much like a newer truck.

Anybody that has one of these trucks will really appreciate how nice it is to apply the brakes after this swap.

The first time I drove this truck I kept saying " I hate the brakes in these old trucks, I think I made a mistake"

Now I love my truck....I have upgraded many things and really enjoy it now....Next up is a 502 with an LS computer and some GM 17" wheels from 2010 that will bolt right up.

The entire brake swap was about $700 bucks in case your wondering.

That's a van master cylinder from 96-2002...those had drums in the back.

Very interested in this.... I ordered front calipers/pads (RockAuto Power Stops) and absolutely hate the drums that won't adjust properly.
I tow a 2100# car on a 20' trailer & braking sucks as I've had to manually adjust the rear shoes twice ..... but that only lasts so long before having to re-adjust. I mostly stop using the trailer brakes..... Lol.

That van master cylinder has an 1 1/8” bore. I’m assuming you had an 1 1/4” bore? That would explain the line pressure increase you saw with the swap. What I’m not sure that explains is why you were seeing such low line pressure on the fronts unless that’s how GM designed these trucks? If that’s the case, that GM designed them with a low line pressure, just swapping out with a smaller bore master cylinder should help.

Are we correct to assume the pressure readings you got were from standing on the brake pedal has hard as reasonably possible?

This is a great thread! Thanks for helping figure this out.

That particular master cyl shows it on another truck that had disc front and rear. A guy on a brake forum said that it was used in both applications and its possible there was a prop valve for the rear drums. Having said that it was not a step bore Master cyl but a true 1"

454cid said:

That's a van master cylinder from 96-2002...those had drums in the back.

Click to expand...

1/8 bore. A 1" inch would have been too much pressure according to some math we did.
The pressure was applying the pedal firm but not standing on it to some extreme degree. But a hard stop is what we had in mind. I can only imagine the factory master cyl which was working had next to NO pressure with a light stop.

bugdewde said:

Very interested in this.... I ordered front calipers/pads (RockAuto Power Stops) and absolutely hate the drums that won't adjust properly.
I tow a 2100# car on a 20' trailer & braking sucks as I've had to manually adjust the rear shoes twice ..... but that only lasts so long before having to re-adjust. I mostly stop using the trailer brakes..... Lol.

Click to expand...

This is the best thing you could do for your truck. I have working on these brakes on and off for 6 months or so trying to figure out why they are so bad. You will be in shock how nice your truck will stop with that conversion. Until I got that master cyl on it was not complete.

andy396 said:

That van master cylinder has an 1 1/8” bore. I’m assuming you had an 1 1/4” bore? That would explain the line pressure increase you saw with the swap. What I’m not sure that explains is why you were seeing such low line pressure on the fronts unless that’s how GM designed these trucks? If that’s the case, that GM designed them with a low line pressure, just swapping out with a smaller bore master cylinder should help.

Are we correct to assume the pressure readings you got were from standing on the brake pedal has hard as reasonably possible?

This is a great thread! Thanks for helping figure this out.

Click to expand...

I have no idea what the factory master cyl had for bore size. I have had several race cars thru the years and I called some people that used to do brake stydies on race cars and he told me what to use for bore size...he is an engineer and was spot on. Also said no residual pressure and you probably wont even need a prop valve.
He was spot on...finding the master cyl took awhile. What was important was the dia. and depth where it mounted. Some would not fit but this one did.