More disc swap Questions (1 Viewer)

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Just did a disc swap using Monte Carlo Calipers along with an aftermarket Prop. valve and 2 lb. residual valve. I left the factory master, prop valve, and as far as I know there isn't a factory residual valve (85' 60). After bleeding the system (I probably pulled about 2 qts. through the system) my pedal is still soft. It will go to the floor and the truck will slow but no where near locking them up. I've tried the prop valve all the way in, out, in the middle, etc. With it all the way out (least amount of fluid to the rear) it's better but still not acceptable.

I'm really hoping for some stupid simple answer on this one as I'm trying to hit the trails for the long weekend but I'm not sure I want to do it with marginal brakes.
 
Howdy! Did you "bench bleed" the master cylinder? It could of drained out/got some air in it. John
 
ill agree on this just being an air in line problem. I remember holding my calipers in all kinds of positions to get all the air to the bleed nozzle. But yours maybe a different location. Also try pumping without letting the pedal come all the way back if the pedal has too much movement it can let air in although this probally isnt the case unless youve adjusted it.
 
If you have the stock MC it might not be pushing enough volume. Swap it out for a 1" bore model and you'll be golden.
 
BTW, it is able to pump up a little which makes me think it could be some air in the system. But after pumping it will bleed down which makes me think there is a leak in the system. I saw one @ the residual valve and tightened it up.
 
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The basic problem is that the rear caliper pistons are too big. Others have made them work, so it is possible.

With such large pistons the hydraulic leverage ratio is off and the pedal will be mushy feeling. The partial solution is a master with a larger bore, but that puts the front hyd ratio out of whack. So then front calipers with enough larger piston(s) must be sourced to restore the total system balance. Finding a rear caliper with a suitable piston size has been a challenge. Most won't care but I want a fixed rather than floating caliper.

P-Valve only affects pressure, no change in volume. https://forum.ih8mud.com/60-series-wagons/256361-comments-brakes-2.html#post4359743

FF or SF rear axle?
SF will allow for slight axle shaft translation, which results in the rotors pushing on the pads, which pushes the pistons in just a bit further. Doesn't take much with a 2.5" diameter piston to require a lot of fluid volume just to get the pads in contact with the rotors. Assuming no other issues, pumping the pedal once should result in a much higher pedal.

A master for a partial drum brake system should have an RPV in the outlet port. On domestic stuff they're hidden behind the sealing cone insert in the port. No idea where Toyota/Aisin puts them. Classically a drum brake RPV is about 10 psi.
 
Yeah I knew the calipers were a bit big. But I have heard of many people that have made them work. I'm trying to figure out if I missed something or just overlooked something simple in the system. I've done a few other rear disc swaps but it's always been on stuff that was already using a larger master. I understand what the prop valve does, unfortunately that early in the morning my brain and fingers weren't on the same page. :lol:


I'm half thinking about throwing a GM master on there off a 1/2 or 3/4 ton so it would have plenty of volume but I can't do that before the weekend.
 
There is a residual valve in your old master, but that won't be causing the issues you are runing into.

You still gotts air in the system somewhere.

I'd pull the old prop valve and the old res valve out and throw them away...

go from there.
 
They are mounted so the bleeder is straight up.

I had a buddy come over tonight and we pulled a little air out of the master. I re-bled and the brakes are definitely better. They won't lock up the tires but neither would the drums. The pedal is still a bit soft but it will definitely work for the weekend.

Thanks for the help extra heads to bounce ideas off of. :cool:
 
Shoot! I'm a little late. I was going to mention the rubber mallet trick just in case you've not heard of it. Sometimes it takes a gentle, rhythmic thumping (NOT wailing on it!) with a rubber hammer while bleeding to get all of the air out of the system.

snip.....
I understand what the prop valve does, unfortunately that early in the morning my brain and fingers weren't on the same page.
10-4!! BT,DT.....
 
They are mounted so the bleeder is straight up.
They are designed to be mounted so the banjo bolt is going straight up into the bottom. That will put the bleeder port right at the top of the cylinder.
 

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