MikeB
Full Access Member
- Joined
- Apr 4, 2014
- Posts
- 1,749
- Reaction score
- 936
- Location
- North Texas
- First Name
- Mike
- Truck Year
- 1969
- Truck Model
- C10
- Engine Size
- 355
Bought a new Raybestos M/C last week and was surprised to see the piston ID was a whopping 36mm. Turns out it's a quick take-up master cylinder with a large piston in the back and smaller 25mm pistons in the front. Here's how it works based on what I've been able to find:
Basically, there are three pistons in a quick take-up master cylinder -- a large one at the rear of the MC and two smaller ones. 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.
I'm not sure when GM started using this type M/C, but turns out it's exactly what my truck has on it now. Also, it looks like power brakes may use a conventional single bore M/C.
Good diagram here: http://www.freeasestudyguides.com/graphics/quick-take-up-master-cylinder.png
Basically, there are three pistons in a quick take-up master cylinder -- a large one at the rear of the MC and two smaller ones. 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.
I'm not sure when GM started using this type M/C, but turns out it's exactly what my truck has on it now. Also, it looks like power brakes may use a conventional single bore M/C.
Good diagram here: http://www.freeasestudyguides.com/graphics/quick-take-up-master-cylinder.png
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