Hypothetical MC question

[Dino]

I was cleaning up the garage and putting parts away when I grabbed the MC I have laying aside to do the front disc brake mod.  It's a Raybestos MC36283 with a 1 1/18" bore.  I realized that may be too big and give it a hard pedal so I may need another.  I think the drum master is a smaller size so I'm wondering if I could leave it on and try just to see what pedal feel I get.  Not to actually drive around with a drum master, but only to test the pedal pressure.  If it is how I want it or close then I can order a disc MC with the correct bore size.

So if I replace the drums with discs and I disconnect the rubber hose at the brake line, will that run of line coming from the master drain?  Can I simply bleed the fronts only to try it out?  Once a new master is on the whole system will be bled.

[Ghoste]

After changing anything in the brake system, I would bleed all four corners always.  It doesn't take long and its safety.  I know you say you don't want to drive around but you still need to know the entire system is okay before you can evaluate a change in a part of it.

[Dino]

After changing anything in the brake system, I would bleed all four corners always.  It doesn't take long and its safety.  I know you say you don't want to drive around but you still need to know the entire system is okay before you can evaluate a change in a part of it.

You make a good point, if not bled correctly I still won't know if the pedal feels right.

Right, no driving.  If I don't remove the front residual pressure valve from the drum MC, the discs will drag anyway.  Besides the reservoir is too small and I would forget to check the fluid level.

I just found out that I've had the new MC longer than I thought so since I can't send it back I may as well bench bleed it and bolt it on.  If the pedal's too hard I'll get another mc.

Well there goes my hypothesis!   

[John_Kunkel]

I'll get a lot of flak for this but I routinely change master cylinders without bleeding the system.

If the MC is properly bench bled and the bench bleeder tubes are left in place while the MC is installed on the firewall, no air will enter the system. The trick is to have somebody slowly depress the pedal as each hard line is attached to the MC, the fluid flowing out of the ports will prevent fair intrusion and will flow into the hard lines as the nut is tightened. Messy? Yes, but not that bad if precautions are taken.

[Dino]

It makes sense that it would work that way but yes I can see how that gets a bit messy.    

[fy469rtse]

what the physical differences in bore size, Dino
could you sleeve this and use the kit to rebuild at the saller diameter for Disk brakes bore size ? John will know this off the top of his head ,
would take me too much time to look it up

[Dino]

what the physical differences in bore size, Dino
could you sleeve this and use the kit to rebuild at the saller diameter for Disk brakes bore size ? John will know this off the top of his head ,
would take me too much time to look it up

I honestly do not know, I thought the drum mc had a bore of around 1" which is why I was hoping I could try it with that master to get the feel for pedal travel.  I still could do so if I remove the RPV from the front line of the master, but if the discs for some reason do not work out and I need to invest in a different brake kit then I'm going all out and get the best I can find.  In other words, that's not going to happen this year. Sooo I would need to put the drum brakes back on so I want to save the mc.

I suppose the disc master I have can be adapted, but I don't want to invest that amount of time and I doubt I have all the tools for the job so I'd rather buy a new mc with smaller bore.  I paid around $40 shipped for the one I have so not a big loss.  Besides, I can always sell it to someone who needs a 1 1/8" bore size.