Double Pumping the Brakes (1 Viewer)

[BadlyBent]

I recently did 4-wheel discs on my 71 FJ40. Used JTO - lousy service. Front bracket not ideal. I'll probably rebuild the front in the future using the mini-truck buildout.

Am having to double-pump the brakes episodically to keep the pedal from depressing too far. No fluid leaks. Bled the lines well. Stops well (and straight for a change) otherwise. No problems with rear-lockup.

Using the stock master with assist. Didn't have that problem with the drums (lots of other problems like leaks, angled stops, etc).

Do I need to rebuild/ replace the master? Is the OEM adequate for 4-wheel discs?

 

[Coolerman]

Are you talking about an OEM master for a 71? If so that master is for DRUM brakes and will have the residual valves which can cause your disks to drag. You need a master designed for 4-wheel disks.

 

[Poser]

Are you talking about an OEM master for a 71? If so that master is for DRUM brakes and will have the residual valves which can cause your disks to drag. You need a master designed for 4-wheel disks.

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.

 

[Coolerman]

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.

I don't doubt that for a second but since a drum brake master moves less fluid per stroke than a disc brake master would that not make one have to pump the brakes more to get a full pedal?
School me on this!

 

[rusty_tlc]

On my front disk conversion all I did was remove the front residual valve and all is well. No noticeable change in the pedal stroke, in fact they may be a little quicker, hard to tell since the brakes were over due for adjustment before the swap.

Wouldn't pressure be more important than volume in a hydraulic system?

 

[Otterav]

Hello,
This issue is hydraulics 101, the diameter of the caliper pistons and the diameter of the mastercylinder are the culprit. If your caliper pistons are too large the mastercylinder will use most of its stroke just taking up the clearances.(your issue) If the mastercylinder is too large your brakes will be hair trigger(not yours) I used FJ60 knuckles and brakes as well as the FJ60 mastercylinder w/ OEM proportioning valve. WOW!!! You might pull your mastercylinder and check the bore for corrosion, if the it is leaking internally you will not get a full stroke volume. You might also try a FJ 60 wagon mastercylinder. If you want a 40 series disc brake master cylinder I have a spare I will sell pretty cheap, make me an offer.
Ola
eric

 

[Otterav]

On my front disk conversion all I did was remove the front residual valve and all is well. No noticeable change in the pedal stroke, in fact they may be a little quicker, hard to tell since the brakes were over due for adjustment before the swap.

All the residual valve does is keep a small amount of pressure in the lines to prevent the brake shoe springs from retracting the shoes all the way back into the wheel cylinder, it would only shorten the stroke since the shoes would not have to travel as far.

Wouldn't pressure be more important than volume in a hydraulic system?

Not really, pressure without volume would only move a small amount, and volume without pressure would not move anything. Hydraulics is a juggling act 'nuff pressure at a sufficient volume to do the work is the order of the day!
Ola
eric

 

[rusty_tlc]

Hello,
This issue is hydraulics 101, the diameter of the caliper pistons and the diameter of the mastercylinder are the culprit. If your caliper pistons are too large the mastercylinder will use most of its stroke just taking up the clearances.(your issue) If the mastercylinder is too large your brakes will be hair trigger(not yours) I used FJ60 knuckles and brakes as well as the FJ60 mastercylinder w/ OEM proportioning valve. WOW!!! You might pull your mastercylinder and check the bore for corrosion, if the it is leaking internally you will not get a full stroke volume. You might also try a FJ 60 wagon mastercylinder. If you want a 40 series disc brake master cylinder I have a spare I will sell pretty cheap, make me an offer.
Ola
eric

I'd be inclined to try Posers suggestion first since it cost nothing except some time and brake fluid. It also agrees with all the research I did before starting my front disk conversion.

 

[Otterav]

I'd be inclined to try Posers suggestion first since it cost nothing except some time and brake fluid. It also agrees with all the research I did before starting my front disk conversion.

I would agree try it, but it will not change anything. If your discs were dragging I would agree, that is all the residual valve does. Your situation is different, you are not getting enough fluid to take up the disc/pad clearance and build braking pressure. I would bet you have master cylinder condition issues, an internal leak due to corrosion will not show externally but when the piston and seals sweep past the corrosion area fluid leaks past and will not build enough volume and pressure to actuate your brakes properly. Do yourself a favor and completely remove the mastercylinder and examine the bore. This will also only take time and brake fluid, if the mastercylinder is corroded you will have your answer. One other thing to check, make sure that your rear brakes are adjusted correctly, if they are loose they will take more fluid to fill the system and actuate.
Good luck and keep us posted
eric

 

[dgangle]

force=pressure X area

change any one of the three, keep the second constant and the third will change.

a larger bore MC does move more volume of fluid, as stated.

 

[rusty_tlc]

I would agree try it, but it will not change anything. If your discs were dragging I would agree, that is all the residual valve does. Your situation is different, you are not getting enough fluid to take up the disc/pad clearance and build braking pressure. I would bet you have master cylinder condition issues, an internal leak due to corrosion will not show externally but when the piston and seals sweep past the corrosion area fluid leaks past and will not build enough volume and pressure to actuate your brakes properly. Do yourself a favor and completely remove the mastercylinder and examine the bore. This will also only take time and brake fluid, if the mastercylinder is corroded you will have your answer. One other thing to check, make sure that your rear brakes are adjusted correctly, if they are loose they will take more fluid to fill the system and actuate.
Good luck and keep us posted
eric

Actually adjusting the brakes has always solved the double pump problem for me with all drums.

 

[BadlyBent]

Yep, original OEM master for the drums.

 

[BadlyBent]

I removed the rear residual only. Should I remove both?

 

[BadlyBent]

I would agree try it, but it will not change anything. If your discs were dragging I would agree, that is all the residual valve does. Your situation is different, you are not getting enough fluid to take up the disc/pad clearance and build braking pressure. I would bet you have master cylinder condition issues, an internal leak due to corrosion will not show externally but when the piston and seals sweep past the corrosion area fluid leaks past and will not build enough volume and pressure to actuate your brakes properly. Do yourself a favor and completely remove the mastercylinder and examine the bore. This will also only take time and brake fluid, if the mastercylinder is corroded you will have your answer. One other thing to check, make sure that your rear brakes are adjusted correctly, if they are loose they will take more fluid to fill the system and actuate.
Good luck and keep us posted
eric

Eric,

I'm running GM cylinders front and back - would a 40 disc master be adequate or should I go to a 60? What's the difference in the bore/ volumes?

 

[Vicm]

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.

Absolutely true.
My drum brake MC works fine pushing 4 Toy Calipers, I didn't even take the residuals out of it. No lockups No brakes draging didn't matter how far I drove.
I even had the truck on jack stands spinning the rear tires at 100km to see if my rear brakes were working properly, I couldn't find any faults with my brakes.
I have made few 160km trips with these brakes.
I am not saying to use Toy parts , but I would say the fault is not with the MC.I hear people have a lot problems from poor adjustmen of the push rod. Which can cause lockups or poor MC bleading.
Vic

 

[Otterav]

Eric,

I'm running GM cylinders front and back - would a 40 disc master be adequate or should I go to a 60? What's the difference in the bore/ volumes?

BB,
I am using basic hydraulic theory, I do not know what the bore of the original mastercylinder, nor the GM calipers. The bore relationships are all that matters, GM, MOPAR, TEQ, ect. as long as you can plumb it together it will most likely work, to a degree. I am betting you have a mastercylinder condition issue, since many other posters say the original MC and GM stuff works ok. I have been an aircraft mechanic for 20+ years and have extensive modding experience, but alas I do not have TEQ specs memorized, nor handy. 1st mechanics lesson, check the simple stuff 1st. 2nd lesson, if you are gonna mod, make sure your components are in serviceable condition(MC?), 3rd, design is easiest and cheapest on paper vs buying hard parts and seeing it it will work. My first bet is your MC condition, but what the heck, my opinion is worth exactly what you paid for it!
keep plugging away, and keep us posted!
eric

 

[Mace]

Third aspect to think about. The drum master cylinder actually puts out MORE force on braking when you switch to disks (why the rears lock up first when you do a RDB conversion)

By increasing the bore size, you are actually reducing the PSI on the caliper..

 

[dgangle]

Third aspect to think about. The drum master cylinder actually puts out MORE force on braking when you switch to disks (why the rears lock up first when you do a RDB conversion)

By increasing the bore size, you are actually reducing the PSI on the caliper..

That is not what I have read and personally found. Drum rears require approx 600-850 psi, disk rears require 1000-1200 psi. The reason the brake bias is off (causing the rear to typically lock up first) is because the GM metric calipers have a piston that is something like 2 1/8" in diamter. F=PXA. Increase the area of the piston, keep the pressure the same and the clamping force of the piston is way more than is required. The solution to this involves reducing the pressure (via an adjustable proporting valve) or reducing the piston diameter, or in extreme cases both. Coleman Racing makes a kit to reduce the caliper piston diameter. I had to do both to get my bias correct when the truck is lightly loaded as in no hardtop, etc.

Back the pressure issue..... I have found that the pressure any MC develops is greatly dependent upon how hard you push on the pedal! Measured at the MC and the calipers, these pressures can vary from several hundred psi to several thousand psi. The secret in any 4WDB conversion is to get the needed pressuress front and rear simultaneously with one push on the pedal. Getting off topic qickly here but fact nonetheless.

Back to the original question of this thread....
If what you have ain't working the only way I know how to start intelligently troubleshooting this is to measure what pressures you have. Stainless Steel Brakes sells a kit for a $50 bucks to do this. If you gotta pump with disks then the fluid is going somewhere, that is assuming there is no air in the system. It may be in the MC. I would suggest starting there. Marlin sells a direct bolt-on 4WDB MC that bolts up to any 40,55,60/62 or 80-series booster plus a whole buch of Celica, Supra, Hilux/minitruck, on and on.

 

[Mace]

That is not what I have read and personally found. Drum rears require approx 600-850 psi, disk rears require 1000-1200 psi. The reason the brake bias is off (causing the rear to typically lock up first) is because the GM metric calipers have a piston that is something like 2 1/8" in diamter. F=PXA. Increase the area of the piston, keep the pressure the same and the clamping force of the piston is way more than is required. The solution to this involves reducing the pressure (via an adjustable proporting valve) or reducing the piston diameter, or in extreme cases both. Coleman Racing makes a kit to reduce the caliper piston diameter. I had to do both to get my bias correct when the truck is lightly loaded as in no hardtop, etc.

Back the pressure issue..... I have found that the pressure any MC develops is greatly dependent upon how hard you push on the pedal! measured at the MC and the calipers, these measured pressuress can vary from several hundred psi to +2000 psi. The secret in any 4WDB conversion is to get the needed pressuress front and rear simultaneously with one push on the pedal. Getting off topic qickly here but fact nonetheless.

You sure you are not agreeing with me???

 

[dgangle]

You sure you are not agreeing with me???

force = pressure X area

In your reasoning more area at the same force results in less pressure? Mathematically impossible!

If you are putting less pressure on the caliper piston (with a MC with a larger bore) it is because you are not pushing as hard on the pedal.