Step Bore Master Cylinder?

[BBShark]

I have seen some S-10 master cylinders listed as "dual" or "step" bore at 24mm/32mm. Does anyne know what the purpose of this is and how they work?

[andrewb70]

I have seen some S-10 master cylinders listed as "dual" or "step" bore at 24mm/32mm. Does anyne know what the purpose of this is and how they work?

High volume at the initial stages of the stroke, then higher pressure as the pedal is pressed further. It was designed for GM trucks that used rather large rear wheel cylinders that needed a lot of volume. I use one on my GTO to get a large volume of fluid to my 6 piston calipers in the front and the small bore generates enough pressure to have manual brakes.

Andrew

[BBShark]

So both front and rear ports see the same pressure at full braking? Sounds like the bigger piston pushes a lot of fluid then the smaller "takes over" to build pressure? Wouldn't this decrease pedal travel also?

I was thinking the two pistons were each seperated. Big piston to rear and small to front so that you would have high pressure/low volume at the front and low pressure/high volume at the rear (drums).

[andrewb70]

I am not really sure how it works from a design point of view, only from a point of functionality. Ultimately it does reduce pedal travel, as well as pedal effort. The design attempts to push a large volume of fluid, with high pressure, and less pedal effort, per given stroke.

Andrew

[Apogee]

The primary bore pushes high volume as stated up to a certain pressure, say 100 psi for discussion sake, then transitions to the smaller secondary bore and begins to develop pressure at a more rapid rate.

The design solves several issues associated with volume and big calipers more than wheel cylinders actually. When CAFE standards for the OE's forced them to design more towards fuel mileage, low drag calipers became the norm. Low drag calipers have square-ish seals that retract the pistons further than necessary to make sure that they don't drag on the rotors which was supposedly good for 1-3% better mileage. Since the pistons have further to travel before making contact with the rotors, the quick take-up MC achieves this with the primary bore.

[BBShark]

I think I can see how that might work. Thanks for the info.

[Tito]

There are some good pics in the middle of this page:
http://www.babcox.com/editorial/bf/bf60133.htm

Basically, there are three pistons in a quick takeup master cylinder -- a large one at the rear of the MC and two smaller ones. Like Apogee says, these are a result of CAFE standards and low-drag calipers, where the caliper pistons actually retract into the calipers a bit on brake release. The QTU M/C applys a high volume of fluid initially using the rearmost piston to push fluid past the seals on the other two pistons. Then the smaller pistons take over and things work normally...

[andrewb70]

Personally I think this is a great MC when used with large front calipers in a manual application. The only drawback is the lack of a deep hole for the pushrod.

Andrew

[speED]

Hi guys,

Quick question, what year S-10 blazer are we talking about?

ED

[andrewb70]

I think mine was for an 85 S10 pick up, with a 4 cylinder and manual brakes.

Andrew

[neki67]

At least some of the 3rd gen Fbodies also use a dual, step size MC. Does the same principle apply or do these lack the quick take up valve?

[speED]

Andrew,

Thank you!

ED

[Skip Fix]

So how did GM keep the pushrod in place with the shallow cup on a manual brake truck?

I fact in recently pulling apart the 81 TA 4WD master I have it looks like there is enough meat to dril the recess on that one deeper. I always though a QTup MC would be good for a aftermarket disc setup.

[BBShark]

Anyone know if the S-10 step bore MC has a residual valve for the back brakes?

[andrewb70]

Anyone know if the S-10 step bore MC has a residual valve for the back brakes?

Not that I am aware of.

Andrew