Double Pumping the Brakes
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Mark W
Yep, I really don't really care that much I guess.
The front and rear circuits of the M/C have the same bore. Therefore they put out the same pressure. depending on the internal configuration and port placement, one circuit can push more volume than the other, but not more pressure.
Drum brake Cruiser M/Cs have a larger bore and push more fluid than the disc brake M/Cs. For the same input pressure a larger bore creates less output pressure... Drum brake M/Cs do not create more braking force... more volume yes, more pressure, no (pressure is what stops the rig, not volume). Normally you need to remove the residual valves from any circuit which pushes disc brakes rather than drums. Many used M/Cs have residual valves which leak down and this leads to folks who are able not remove the valves and have no problems.
I have had good luck using inline residual valves to keep disc brake systems nice and crisp. You can us a residual on disc, but you use a lower pressure. Summit sells these and I'm sure you can get them other places as well.
The reason for proportioning valves on factory systems is that drums require lower pressure (but more volume) for the same braking force as discs. Puls the rear axle has less traction under braking so the braking force has to be further reduced. When the common GM caliper conversion is performed on the rear axle, the situation is additionally complicated by the larger bore of the GM unit. This creates greater braking than the front Toyota caliper with the same pressure. Reducing the pressure for the rear brakes is required to make things behave.
Back to the original question... I have had similar problems with a JTO kit on a rig that I've worked on a bit. It seems that the GM calipers require more fluid than the Toyota M/C can push. Even the 1 inch bore drum brake M/C. I haven't done any math to verify this though.
The braking is good, but only at the very end of the stroke. If I could get a bit more stroke out of the pedal/master cylinder, I'd be happy with it, but as it is, a panic stop is pretty iffy I'll probably eventually try swapping in a GM MC as well. Although I'll be trying the 2 pound residual valves first.
Mark...
Drum brake Cruiser M/Cs have a larger bore and push more fluid than the disc brake M/Cs. For the same input pressure a larger bore creates less output pressure... Drum brake M/Cs do not create more braking force... more volume yes, more pressure, no (pressure is what stops the rig, not volume). Normally you need to remove the residual valves from any circuit which pushes disc brakes rather than drums. Many used M/Cs have residual valves which leak down and this leads to folks who are able not remove the valves and have no problems.
I have had good luck using inline residual valves to keep disc brake systems nice and crisp. You can us a residual on disc, but you use a lower pressure. Summit sells these and I'm sure you can get them other places as well.
The reason for proportioning valves on factory systems is that drums require lower pressure (but more volume) for the same braking force as discs. Puls the rear axle has less traction under braking so the braking force has to be further reduced. When the common GM caliper conversion is performed on the rear axle, the situation is additionally complicated by the larger bore of the GM unit. This creates greater braking than the front Toyota caliper with the same pressure. Reducing the pressure for the rear brakes is required to make things behave.
Back to the original question... I have had similar problems with a JTO kit on a rig that I've worked on a bit. It seems that the GM calipers require more fluid than the Toyota M/C can push. Even the 1 inch bore drum brake M/C. I haven't done any math to verify this though.
The braking is good, but only at the very end of the stroke. If I could get a bit more stroke out of the pedal/master cylinder, I'd be happy with it, but as it is, a panic stop is pretty iffy I'll probably eventually try swapping in a GM MC as well. Although I'll be trying the 2 pound residual valves first.
Mark...
dgangle
total rice
Are you sure this is correct? The 4 wheel drum brake MC I bought at one time for my 08/75 was 15/16" bore, IIRC while the 4 wheel disc MC I bought from Marlin Crawler a year or so ago has a 1" bore. This seems to contradict what you say but I could be mistaken.
Mark W
Yep, I really don't really care that much I guess.
Not sure about aftermarket M/Cs, but the OEM FJ40 and FJ60/62 disc brake M/Cs are smaller than the FJ40 drum brake M/Cs. 13/16 and 7/8s on the disc brake M/Cs that I can recall off the top of my head. 1 inch on the later drum brake M/Cs,not remembering about the early ones right now.
Mark...
Mark...
FWIW I had the JTO kit with the metric GM calipers and it worked o.k. with my 70 drum master. I have since put the standard GM 1/2 ton calipers from an 86 K5 blazer and it stops better, less pedal travel. It does not make sense cause the 1/2 ton GM caliper has a larger piston than the metric caliper but it seems to use less fluid to make it move. I was not impressed witht the JTO bracket, and alot of the stuff for the conversion was bs. The bolts they sent were to long, bottomed out, you had to delete the felt wiper ring on backside of hub, and the discs they sent required a small amount of material be removed from outer edge of hub.
IMO put a 60 in.
IMO put a 60 in.
you are actually oversimplifying things...
There is pressure from your foot
Volume of the master
Volume of the caliper
And pressure on the piston (in the caliper)
If you change one, at least one other aspect has to change..
If you switch to a GM caliper and stick with the stock master, you will easily be able to lock up the brakes.. = more braking force...
F=M*A only covers half of the system.
I have to go dig a hole for an apple tree.. I'll get the actual calculations later.
There is pressure from your foot
Volume of the master
Volume of the caliper
And pressure on the piston (in the caliper)
If you change one, at least one other aspect has to change..
If you switch to a GM caliper and stick with the stock master, you will easily be able to lock up the brakes.. = more braking force...
F=M*A only covers half of the system.
I have to go dig a hole for an apple tree.. I'll get the actual calculations later.
It is hard to make sense out of this. I'm not sure how the internals of the two-circuit work ,but I imagine that they are like two master cylinders stacked on each other sharing a common pushrod. Anyway, if the bores are the same and they are connected by a common rod, then they will put out the same pressure and volume. Volume is determined by the area and stroke of the bore. V=AxL As was said earlier MC pressure is determined by the amount of pressure put onto the pedal.
The MC can have a pressure multiplying effect on WC as long as the bore of the WC is bigger than the bore of the MC. It is pounds per square inch. So if you have an area of 1" at the MC and you put 1 pound of pressure on it and you have a WC with 2" of surface area you will get two pounds of pressure. This is the way hydraulic jacks work also. This is one of the reasons why brakes have to ride so close to their braking surface, they take up more volume than they MC can put out, because they are bigger. Even if the bore of one WC is the same you have two per wheel.
I think when people think that their rear drums are locking better, than their discs they are forgeting the mechanical advantage that the drums have. Drums are designed to push into the drum as the brake is applied where the pads in a disc system are directly proportional to the pressure behind them.
So let's see if I can apply this all to clear things up.
You have to apply more pressure to discs than to the drums to overcome mechanical advantage of drums.
You have to make sure the stroke of all WC's does not exceed the volume of the MC. This is done with adjustment on drums and back pressure on discs.
I am unclear on the way a proportioning valve works. If it only serves to slow the fluid down, than you still have the pressure problem in the drums which as far as I know can only be solved by bigger disc pistons or a bigger MC chamber for the drums.
You also have the problem of having more weight up front which again you would have to subtitute with bigger bores.
You can remove the residual pressure valves and use a dual circuit drum brake master cylinder with a four wheel disc brake system.
When using Toyota calipers up front and GM in the rear, it works fine. There are more than a couple 40 series running around out there with a drum brake master cylinder supplying fluid to a four wheel disc brake system.
yup, the drum master cylinder also works fine with Wilwood calipers and in the front and the GM calipers in the rear
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Mark W
Yep, I really don't really care that much I guess.
The plungers in the M/C have a spring between them and then a shorter solid rod. the rod only bottoms out after the spring is compressed. Using a system like this you can generate more volume (more travel) in one circuit than the other. Pressur stays the same.
Larger surface area (bore diameter) creates lower psi for the same amount of foot pressure on the pedal. Assuming that the leverage of the pedal stays the same.
A larger bore in the slave cylinder (caliper piston) or multiple slaves will create more overall force with the same amount of psi delivered from the master. If somehow you had less surface area in the slave than the master, then of course you would have a reduction in force.
Volume of the master and volume of the slave(s) only comes into play if you run out of stroke at the master (this is what seems to be happening with the four GM caliper setup). unless you can't push sufficient volume to move the slaves as much as needed, then it is only pressure that you are concerned with. And volume of the two cylinders doesn't affect this. Only surface area of the piston (bore of the cylinder).
Proportioning valves reduce pressure, they don't just slow the delivery of volume.
Mark...
Larger surface area (bore diameter) creates lower psi for the same amount of foot pressure on the pedal. Assuming that the leverage of the pedal stays the same.
A larger bore in the slave cylinder (caliper piston) or multiple slaves will create more overall force with the same amount of psi delivered from the master. If somehow you had less surface area in the slave than the master, then of course you would have a reduction in force.
Volume of the master and volume of the slave(s) only comes into play if you run out of stroke at the master (this is what seems to be happening with the four GM caliper setup). unless you can't push sufficient volume to move the slaves as much as needed, then it is only pressure that you are concerned with. And volume of the two cylinders doesn't affect this. Only surface area of the piston (bore of the cylinder).
Proportioning valves reduce pressure, they don't just slow the delivery of volume.
Mark...
Bodean
Hot Damn
A friend of mine recently bought the M/C that JTO sells for disc brake conversions and had better luck with that. It cured a lot of the locking up issues in the rear. The M/C looked like an 80 series M/C. Anyone know?
Later,
Later,
dgangle
total rice
From personal experance, you need to change your master. I have 70 FJ40, stock with 4 wheel drums. I converted to 4 wheel disc all at once. Front are Toy mini and rear are from an El Dorado. The stock master did not work for me. The Mini Truck master did not work for me either. I am currently using the 4 wheel disc master from an 80 series LC. Works great! Just get a new master from you local Toyota Dealer (comes complete with res), you will be happy you did!
Shawn
Shawn
Where does the pressure bleed off to, in a normal hydraulic system the excess pressure returns to tank but propotioning valve has no return, where does the excess pressure go
dgangle
total rice
A proprtioning valve is nothing more than a regulator. It has a spring that pushes against one side of the diaphram. The outlet pressure of whatever you are trying to regulate tries to match it's outlet pressure against the spring pressure. See
http://auto.howstuffworks.com/master-brake2.htm
http://auto.howstuffworks.com/master-brake2.htm
Mark W
Yep, I really don't really care that much I guess.
In my experience, I have not found any of the dual circuit Toyota master cylinders that will not work fine with Toyota disc up front and GMs in back (or Toys in back. You need to pull the residual valves (unless of course they are not working anyway) and you almost always need to install a proportioning valve.
But the maser cylinder does not need to be changed.
I have less experience with the conversion using 4 GM calipers. With this setup it appears that you do need to change the M/C to one which pushed more volume.
Mark...
Mark W
Yep, I really don't really care that much I guess.
Where does the pressure bleed off to, in a normal hydraulic system the excess pressure returns to tank but propotioning valve has no return, where does the excess pressure go
Volume would need to be bled off through a return line if we were talking about an open system in which fluid circulated (Hydraulic) and we were trying to reduce flow. But we are talking about a closed system with no circulation (more properly called Hydrostatic) in which we are seeking a reduction of pressure.
Reducing the volume would lessen the movement of the slaves, not the pressure exerted by them. A proportioning valve limits the force that the slave exerts, not how much it moves.
Mark...
Mark W
Yep, I really don't really care that much I guess.
Some of the earlier Corvettes had single cylinder master and 4 wheel discs. Pedal pressure is much higher without the booster.
Other than that it is just a matter of how much pressure you can build and how much volume you can push. My first thought would be that the stock early model Cruiser single circuit master won't be up to the task. But I've never tried it or really given it much thought either.
Mark...
