Blazer brake upgrade - Proportioning valve change required?

[Clarkwb100]

I upgraded the factory single piston brakes to the S-10 / Blazer dual piston assembly a couple years ago. The spindles were from a 2001 2wd Trailblazer, all the rotating assembly, the brake rotors, pads and brake lines were replaced at that time. The master cylinder was replaced with a stock 1987 El Camino unit prior to this major replacement.
I did NOT replace the proportioning valve.
Rear brake drums and pads were replaced with stock GM 1987 El Camino units.
The difference after replacing the single piston brakes was tremendous. But the dual piston brakes have always seemed "touchy". The front brakes lock up quickly now. Even at low speed in parking lots they will lock up if not careful. Inspection of the rear linings shows them working.
How do I get rid of this "touchiness". Replace the proportioning valve? Is this the fundamental problem? What brand & part # is recommended?

Your thoughts and guidance is appreciated.

 

[abbey castro]

I did the front blazer conversion without changing the proportioning valve and I did not have any brake lock-up problems. I did change it to a disk/disk one when i did the rear Camaro disk swap. I have no idea why the front would lock up. Got the disk/disk valve from CPP.

 

[88AZSS]

I did the Blazer swap as well and didnt change the proportioning valve and do not have the touchiness that you described. I did rebuild the rear brakes and have all new pads/shoes front and back. The rear has aluminum drums but I doubt thats the difference. That beind said, I dont have much in ways of suggestions on what to look at next to solve your issue.

 

[403Olds]

I did the blazers and the rear manual wheel cylinders at the same time, no other changes or issues.

 

[ELCAM]

Sounds like non quick take up calipers being driven by a quick take up master cylinder.

 

[MrSony]

Sounds like non quick take up calipers being driven by a quick take up master cylinder.

what does that mean

 

[Built6spdMCSS]

Yeah that's what I'm thinking, it's the master cylinder.

 

[69hurstolds]

I upgraded the factory single piston brakes to the S-10 / Blazer dual piston assembly a couple years ago. The spindles were from a 2001 2wd Trailblazer, all the rotating assembly, the brake rotors, pads and brake lines were replaced at that time. The master cylinder was replaced with a stock 1987 El Camino unit prior to this major replacement.
I did NOT replace the proportioning valve.
Rear brake drums and pads were replaced with stock GM 1987 El Camino units.

It appears the bolded part may be the issue. The stock G-bodies of the 80s had stepped bore masters which indicated they were quick take-up style. Basically pushing a bit of extra fluid to the front disc brakes as soon as you started to move the pedal (that's the theory anyway) so the calipers move further, faster. The D154 stock calipers were made with a beveled seal which pulled the piston back a hair more to reduce drag so they could brag that it helped gas mileage (probably by 0.002 mpg). This meant you have to MATCH the low-drag calipers with the quick take-up master cylinder. If you use that type of master without the low-drag calipers, you'll get the results you describe because it's putting more fluid out faster. That rear-most take-up piston is 1.25" diameter and shoves a lot of fluid fast. Which is the point.

Practically all 80s GM cars used the step-bore masters. About an inch or so prior to the mounting flange, the diameter gets larger. So it's easy to tell about the master cylinders.

As for distinguishing the calipers, not so much by looking at it. However, if it uses metric bleed screws, it's low-drag. The key point is, if it's a factory 1980-88 A/G-body caliper, it's 99.9% sure of being low-drag. Nice thing about one of the options you can get for the factory-style replacements is the zinc-plating ACDelco has. Paint them high heat cast iron and they shouldn't rust for a LONG time if you want a true, factory-style appearance.

Wildwood does have a bolt-on, angled, non-stepped bore GM setup with adjustable valves. Just pick a bore size. It should bolt right up to the factory booster. But rather pricey.

https://www.wilwood.com/PDF/Flyers/fl534.pdf

Sampling. 1" bore kit, burnished. $436.91

https://www.summitracing.com/parts/wil-261-16962-p

 

[Clone TIE Pilot]

It appears the bolded part may be the issue. The stock G-bodies of the 80s had stepped bore masters which indicated they were quick take-up style. Basically pushing a bit of extra fluid to the front disc brakes as soon as you started to move the pedal (that's the theory anyway) so the calipers move further, faster. The D154 stock calipers were made with a beveled seal which pulled the piston back a hair more to reduce drag so they could brag that it helped gas mileage (probably by 0.002 mpg). This meant you have to MATCH the low-drag calipers with the quick take-up master cylinder. If you use that type of master without the low-drag calipers, you'll get the results you describe because it's putting more fluid out faster. That rear-most take-up piston is 1.25" diameter and shoves a lot of fluid fast. Which is the point.

Practically all 80s GM cars used the step-bore masters. About an inch or so prior to the mounting flange, the diameter gets larger. So it's easy to tell about the master cylinders.

View attachment 232969

As for distinguishing the calipers, not so much by looking at it. However, if it uses metric bleed screws, it's low-drag. The key point is, if it's a factory 1980-88 A/G-body caliper, it's 99.9% sure of being low-drag. Nice thing about one of the options you can get for the factory-style replacements is the zinc-plating ACDelco has. Paint them high heat cast iron and they shouldn't rust for a LONG time if you want a true, factory-style appearance.
View attachment 232975

Wildwood does have a bolt-on, angled, non-stepped bore GM setup with adjustable valves. Just pick a bore size. It should bolt right up to the factory booster. But rather pricey.

https://www.wilwood.com/PDF/Flyers/fl534.pdf

Sampling. 1" bore kit, burnished. $436.91

https://www.summitracing.com/parts/wil-261-16962-p

Is there a particular non stepped bore master cylinder that easily replaces the stock stepped master cylinder?

 

[69hurstolds]

Is there a particular non stepped bore master cylinder that easily replaces the stock stepped master cylinder?

You mean cheap? I don't know. Didn't do much research on it since I prefer to stay factory stock whenever possible. I've never did any brake swapping with non-OEM type equipment on my cars. I would IMAGINE that if you look at say, a 1979 master cylinder, it shouldn't be a fast take-up since low drag didn't raise its ugly head until somewhere in 1980 model year. Not sure if it's a bolt-on for later models or not. May be able to adapt the fittings or put new SAE fittings on the brake lines? I'm not a brake modification expert, so I would take anything I say on this with a grain of salt. I do know you shouldn't mix low-drag and regular components, though. It MIGHT be possible to swap out the A-body 78-79 booster as well, since the master cylinder mating bore is smaller, the mounting hole may be as well. Again, I don't know, as I've never mixed and matched. All these are different part numbers, obviously. But I would imagine they would physically fit a later G-body with minimal modification, if any.

Just wild-azz guesses.


For reference, here's a 79 NON- fast take-up master cylinder: