master cylinder swap

[Wrecked1982]

can i swap my old master cylinder on my 85 k35 for a new one with the plastic reservoir? will it fit?

[Keith Seymore]

Maybe.

Some of the differences in master cylinders that will make them not fit are:

1) Quick take up vs non quick take up - affects the size of the part that goes in the booster

2) Port locations - inboard vs outboard

3) Flare type - English vs metric

4) Port location - ie, front reservoir feeds front brakes vs front reservoir feeds rear brakes

5) Reservoir size vs hood clearance - probably not an issue on a truck

Lastly - this doesn't affect fit but does affect function - is the master cylinder bore diameter.

Once you take a look at some of these criteria you'll have a better idea. There not all show stoppers but can make simply bolting on a new m/c a little more tricky.

K

[overtimeracing]

Or...stick it on there and see if it fits...lol

[Keith Seymore]

Here's the brake system release chart for an '87 V30 (I knew these would come in handy some day) -



Looks like if you can find a 14066425 (broadcast code "FT") you should be all set (assuming it's plastic/aluminum).

K

[Keith Seymore]

I should mention the reason I know all this is because, many years ago I wanted to replace the cast iron m/c on my Chevelle with a lightweight aluminum/plastic m/c.

After shopping through the charts I chose several that looked pretty good (an early S truck version, C/K truck, an M/L van m/c, and a D car limo).

I stumbled across several of the differences I noted as I tried this. Many of them resulted in a strange pedal feel, where the engagement was way low (pedal almost to the floor, but ok once the brakes came on). The M/L van version seemed to work the best, as far as brake pedal feel and stopping ability, but the reservoir was so small (and angled) that I had to jack the back of the car up to top off the fluid.

It eventually failed (lost brakes) so in the end I went back to my old cast iron unit and haven't had a problem since. It might have been related to the line-loc, which I removed about the same time, but I never revisted the issue to find out for sure.

My $.03 -

K

[Pyrotechnic]

I wanted to upgrade to plastic aluminum unit on my 77, mainly because it seems like the plastic reservoirs seal up better and it would save some weight.

It turned out that all the aluminum ones were the stepped bore style for quick take up front brakes. From all the browsing I did through parts catalogs, it seems like they never made them for older trucks.

I would love to find an aluminum/plastic MC with a 1-1/8 bore that would fit the truck and work correctly but I haven't had any luck so far.

[Wrecked1982]

ahhh i forgot ti mention that i wanted it to fit on my hydro boost. the main reason i want to change it is just the modern look and they dont rust .sadly i work with my truck and we have DAMN WINTER in cold canada . as far as if it brakes more or not.. its a tow truck so its got to stop lol when hauling pick ups or vans.maybe newer ones had it like the 1988 tru ....91 i think ?when they became they other modele (im talikng about 1 ton duallys)

[Pyrotechnic]

I looked at the rock auto parts catalog, and it seems that all the 1-5/16" bore hydroboost master cylinders up to 1991 are cast iron. Well, at least whatever new replacements are available now.

As far as making a different one work, all the GM aluminum master cylinders I see in catalogs have a stepped bore for the quick take up feature. It's supposed to pull the pads away from the rotor more for better fuel economy. Problem is, they need the matching quick take up calipers and they won't work properly with standard calipers. Not only that, you'd have to have one with the correct 1-5/16" bore size. I'm going to guess that hydroboost trucks never came with the quick take up feature.

The one from 90's 1 ton trucks have an aluminum plastic design with a non stepped bore and the right fitting sizes, but they are 1-1/4" bore. I'm not sure how the large bore would affect they way your brakes work or if it would even fit into the older hydroboost unit.

I'm going through the same thing now with my 77 half ton which is before the quick take up feature, and I couldn't find anything from GM that will work. Today your thread inspired me to do some more looking and I stumbled upon an aluminum MC off a Dodge that just might work.

[INSIDIOUS '86]

Can somebody explain the quick take up feature? I don't want to start a third master cyl thread for the day...lol
Posted via Mobile Device

[Wrecked1982]

i wasnt aware of the take off feature but it makes sens, master cylinder equiped with take off feature means when letting go the brake it will make the caliper piston come back in just a little bit to avoid a bit less friction. normal MC just release the pressure when not aplying brake so the rotor/drum turn again free but still touching a bit the brake pads. i think im right others will correct me if om wrong

if that dodge mc works i might try it when i get time.

[Keith Seymore]

Originally Posted by Wrecked1982

i wasnt aware of the take off feature but it makes sens, master cylinder equiped with take off feature means when letting go the brake it will make the caliper piston come back in just a little bit to avoid a bit less friction. normal MC just release the pressure when not aplying brake so the rotor/drum turn again free but still touching a bit the brake pads. i think im right others will correct me if om wrong

if that dodge mc works i might try it when i get time.

Correct.

So - you could add the quick take up master cylinder and not replace the calipers and it would work, but you simply wouldn't be taking advantage of the feature.

The problem is that the larger rear housing on the QTU m/c will not fit your booster - preventing it's installation in the first place.

K