My Rear Disc swap from drums observations

[John Cunningham]

Your exactly right about overthinking however I am doing it to understand how the wrong master cyl works so well......I am thinking once I throw a guage on the rear discs I am going to see something between 600 and 1000 pounds to the rear and around 1500 to the front. ( a sorta built in prop valve)

I found out the factory non adjustable prop valve is a bout a 60/40 bias. So if the rear discs check out to be about 1000 pounds then its about a 30% reduction in pressure like having a built in prop valve set at that pressure because of the step bore master cyl. That pressure may be perfect for average conditions. I certainly dont think there is any residual pressure so I forgot about that until I see some evidence that is different.

If your towing a large trailer it would be nice to apply more pressure to the rear brakes........The factory prop valve is not for all conditions and an empty truck bed versus a fully loaded bed would require a different bias and the factory does not compensate for that...they take a shot at average conditions and leave it alone so nobody fools with it. I believe some guys are happy with the drums because they have empty beds and it does not matter. However that all changes if you want more pressure to take advantage of the discs and loads.

But because I am old racer I want to know how pressure is hitting the rear discs. If I see something less that 600 pounds I will change the MC...if its around 1000 psi I will leave everything as is and consider itt good because who knows if you change the MC and set up a prop valve you might be matching what you already have.

It took me awhile to understand this but now I get how and why he said you dont have to change the MC..he just did not understand that its like having a prop valve without having one. Of course everybody says you need more fluid volume but I am ignoring that now for the moment too

 

[R422b]

Good work i am looking foward to hearing the results.

 

[letitsnow]

Not sure if this is any help, but when I had the stock rear drum brakes (adjusted correctly), I had to turn my willwood proportioning valve so that the rear brakes had the least amount of fluid, otherwise the rear wheels would lock up too soon. Now that I have the rear discs I had to set the proportioning valve so that the rear brakes have nearly the most amount of fluid to get the rear wheels to lock up.

 

[John Cunningham]

Not sure if this is any help, but when I had the stock rear drum brakes (adjusted correctly), I had to turn my willwood proportioning valve so that the rear brakes had the least amount of fluid, otherwise the rear wheels would lock up too soon. Now that I have the rear discs I had to set the proportioning valve so that the rear brakes have nearly the most amount of fluid to get the rear wheels to lock up.

Thats an interesting observation and I need to ponder on this information....so you did not change the MC? I think we would be better off with not saying the amount of fluid because that is about volume........the prop valve may cut some volume but it is about pressure I think. The disc requires more pressure than a drum to work properly to achieve the same results.
Drum : More fluid and automatic will get less pressure if the MC is a two bore
Discs: Same MC will get less pressure automaticly because of the larger bore.

That would make sense if you did not change the disc drum MC...you need more pressrue to make the disc function and really you were only getting about 1000 PSI at the most with the prop valve all the way open.

So with your prop valve turned back for the drums your disc brakes were getting hardly anything for pressure at the drum brake prop setting..hence the reason you had to open it all the way up and this is correct.

So this is basic proof you did not change the MC

 

[John Cunningham]

Thats an interesting observation and I need to ponder on this information....so you did not change the MC? I think we would be better off with not saying the amount of fluid because that is about volume........the prop valve may cut some volume but it is about pressure I think. The disc requires more pressure than a drum to work properly to achieve the same results.
Drum : More fluid and automatic will get less pressure if the MC is a two bore
Discs: Same MC will get less pressure automaticly because of the larger bore.

That would make sense if you did not change the disc drum MC...you need more pressrue to make the disc function and really you were only getting about 1000 PSI at the most with the prop valve all the way open.

So with your prop valve turned back for the drums your disc brakes were getting hardly anything for pressure at the drum brake prop setting..hence the reason you had to open it all the way up and this is correct.

So this is basic proof you did not change the MC

One more thing......if you had changed to the disc disc MC you would not have been able to keep the prop valve all the way open setting.that would have given you full pressure or about 1500 pounds to the rear discs and you would have cut the pressure back on the prop valve. You only need that kind of pressure where the weight is.......like the front. Unless of course your bed has a 3000 pound camper in it.

 

[Gibson]

Drum : More fluid and automatic will get less pressure if the MC is a two bore
Discs: Same MC will get less pressure automaticly because of the larger bore.

Well, kinda,, the first part, yes, in the step bore master, the larger piston supplies a large volume of fluid to the rear wheel cylinders to expand them, (it "moves" fluid,) then, after the shoes are expanded against the drums, the larger bore exerts less pressure to the fluid column, as drums don't need as high of pressure..
The small piston moves almost zero fluid to the front discs, but transfers a higher pressure.
At slower speeds and gradual stops, a heavily loaded bed can use more braking in the rear,, but when you hit a panic-stop from higher speeds, a good sized proportion of that weight shifts to the front anyway,, and the higher the CG of the weight the more it shifts.
We've all seen big-rigs carrying many tons lock-up the rears trying to stop.

Edit,, we've had a lot of talk going back-an-forth,, but really, one of the best things our trucks have is ABS,, with a good ABS unit you can cram all the pressure you want into the system and let the ABS do its job,,.

 

[John Cunningham]

Your spot on the ABS.............I think those big rigs are locking up the fifth wheel part of the rig hence the sideways look you always see in an accident. Your probably right to exten that the load wants to still shift forward..........so the question is do you even really need a prop valve? Probably if your carrying no load in the pickup bed if that is what you have.

I think I will see and understand more after I see how much pressure I have to the rear with the wrong master cyl.

I looked up the master cyl on Rock auto a Delco part number shows 1.125 Bore...for my mater cyl...does not say what the larger bore is?

The 1.25 Bore will give more pressure but the pedal wont be as good

I also looked up the Master cyl for like the 2000 model truck with 4 wheel discs.......they increased the bore size to 1.5 bore...which is fine because of the power brakes.

I think the 1.5 bore because of the hydroboost will give more than enough pressure and give a better pedal?

I think my pressure test will show 1000 psi or less........in that case what do you think?

I am not taking advantage of my discs?

 

[someotherguy]

Not sure if this is any help, but when I had the stock rear drum brakes (adjusted correctly), I had to turn my willwood proportioning valve so that the rear brakes had the least amount of fluid, otherwise the rear wheels would lock up too soon. Now that I have the rear discs I had to set the proportioning valve so that the rear brakes have nearly the most amount of fluid to get the rear wheels to lock up.

Oh now quit telling stories, everyone knows those drum brakes are totally ineffective. No way you were locking them up.

In this thread I see an odd combination of a whole lot of guesswork and some willingness to do actual research, and input from one guy that actually seems to understand brake systems.

When I see things like misunderstanding the application and role of a proportioning valve (or as correctly pointed out, "combination valve") and the failure to retain parking brake functionality, wow, man, I just.. I.. I'm out. Y'all have fun.. brake systems aren't really for amateur experimentation and guesswork, IMO. Not if you're testing the results on public roads.

Richard

 

[sewlow]

An observation.
Not using the drum e-brake "ever" means that the shoes aren't self-adjusting themselves as they should when the e-brake is set.
These trucks do have somewhat of a prob in that area, though.
GM designed that ratcheting system poorly. The notches on that star-wheel adjuster are too large which means that the shoes to have to be worn down a fair amount before they do self-adjust, resulting in poor brake performance & pedal feel until they do. ...and even then, the shoes do not get a proper amount of adjustment via the self adjusters due to the tooth size.
Top that off with GM's redsign of the brakes with no adjustment tool window on the backing plate in order to be able to adjust the shoes manually, results in having to physically remove the drums so as to adjust via trial-&-error.
Even the old trick of driving in reverse & slamming the brakes on will not get the shoes to self-adjust properly as they do with the older design. The over-sized notches on the star wheel is mainly to blame for that, but the redesign which put the adjuster at the top of the backing plate, instead of the old design where it was on the bottom, also has a finger in that pie.
Want the drum brakes to work properly & consistantly? Pull those drums & manually set them up with the proper drag that they require every couple of thousand miles or so, depending on your driving style.
Yea. GM dropped the ball on the redesign, but when properly set-up, they do work as they should.
While discs are much simpler & easier to maintain, the drum brakes on these trucks are not as problematic as some claim they are, just as long as they are properly adjusted. That requires diligence, which some owners don't seem to want to, know how to, or are even aware of.

That being said, I am putting discs on my '98. Purely for ease of maintenance & aestheics. But, they will be a properly designed set-up, with an adjustable proportioning valve, plus an E-brake. None of this cobbled together, fingers crossed & hope it works guessing game install.
Brakes are not the area to be doing that kinda stuff. There's responsibilty involved here. Not only to yourself & your family, but to every other vehicle on the road & the inhabitants within.

I'm with someotherguy on this one. Way too much guessing & uninformed non-knowledgeable error by trial experimentation for me.

 

[Crookedaxle]

No residual pressure observed see my post

How exactly do you determine that there's no 10psi residual pressure by cracking a fitting? Do you expect the brake fluid to spray out all over the place? Half of a drop of brake fluid probably would be enough to release 10psi of pressure in a brake line where there is no air or opposing force on the line. What else did you expect?