55.2-59 Keeping Pedal pressure with under-cab MC

[daveshilling]

Gents,

Pardon me while I have all these QnA open simultaneously...

So on the 59 Stepside (Jackalope), the power disc/disc aftermarket Master Cylinder is under the cab, and the factory pedal sat very high...like you need to lift up your entire leg. Del cut the pedal down for me many moons ago, but I fear that the mechanics of the OEM pedal require that all original pedal travel be present... as it stands, the brakes are not great...They've been bled multiple times with no change.

I don't want to encounter this issue with Lucky the Trucky (58 Step build in progress) and I considered a firewall mount instead, but I love the clean firewall too, so I'm hoping there's a way to fix Jackie and carry that over to Lucky as well when I choose a Master Cylinder setup for him too.

How are your brakes on Disc/Disc Task Force under-cab MC setups? Rock Solid? Any tricks? Should I just go firewall and get a new pedal?

Safety > Sexy.

[Foot Stomper]

If your drum brakes aren't set up properly, your pedal will travel a farther distance to get the shoes to contact the drum. They need to be adjusted manually and regularly unless they have the auto adjusters which many don't have.

Bleeding the brakes can be a bit tricky if you don't know exactly what you're doing. If you are not 100% sure, then have them bled professionally.

My stock MC and drum set up has a very firm pedal and very efficient stopping ability. The system was designed correctly and is very reliable and effective when working as designed.

[daveshilling]

Originally Posted by Foot Stomper

If your drum brakes aren't set up properly, your pedal will travel a farther distance to get the shoes to contact the drum. They need to be adjusted manually and regularly unless they have the auto adjusters which many don't have.

Bleeding the brakes can be a bit tricky if you don't know exactly what you're doing. If you are not 100% sure, then have them bled professionally.

My stock MC and drum set up has a very firm pedal and very efficient stopping ability. The system was designed correctly and is very reliable and effective when working as designed.

Thanks for the reply. Both trucks are a custom disc/disc setup, with D52 in front, gen 3 Camaro rear for the "Jackalope", and C4 vette front/rear in "Lucky"

[Foot Stomper]

Originally Posted by daveshilling

Thanks for the reply. Both trucks are a custom disc/disc setup, with D52 in front, gen 3 Camaro rear for the "Jackalope", and C4 vette front/rear in "Lucky"

Well then, I guess frankenstein is your issue then. Running discs with a factory MC is a big mistake.
Cheers

[MiraclePieCo]

Power brakes? This is a potential issue:

https://techtalk.mpbrakes.com/how-to...r-cylinder-gap

[1project2many]

In any brake system the fluid in the wheel cylinder or caliper will try to toward the lowest point in the system. If the wheel end is higher than the master cylinder fluid will attempt to flow into the master when brakes are not applied. To some extent the springs in the wheel cylinders resist this by applying outward pressure to the cups. This is not the same in a disc system as most calipers do not have springs behind the pistons. Draining fluid can cause a slight vacuum on the piston which can cause the piston to retract slightly. The result is a long pedal travel before brake application.

There are a number of considerations to help reduce or eliminate this tendency. Trying to keep all brake line runs "downhill" from the master helps but is nearly impossible in lowered trucks with floor mounted masters. Running the brake lines up and then down at some point between wheels and master can help but ultimately if the master is lower fluid will still try to drain out of the wheel ends. So the final step is to ensure there are check valves built into the system.

The OE master cylinder has a check valve installed. this page shows a master of very similar construction to the stock '57 and clearly calls out the check valve, while this rebuild kit photo clearly shows the check valve. With the valve in place when the vehicle is in OEM configuration, it only makes sense to include one in the new design. In some cases a builder will buy "residual pressure" valves and install them inline. In other cases a master cylinder can be selected which contains check valves. Either way, you need a check valve to ensure the system works well.

[dsraven]

if using disc/disc then you will need a residual valve in the front and rear lines and a disc/disc master cylinder that is matched to your calipers as far as displacement is concerned, like you wouldn't want to use a disc brake master from a volkswagen beetle on your set up because it would take a full stroke of the brake pedal just to fill the calipers enough to push the pads out and touch the rotors. the piston on the master cylinder is waaay smaller so it displaces waay less fluid per stroke. you can't use a drum brake master for a disc brake set up because the drum brake master will usually have the residual valves built into the outlet ports. in other words, you need a master cylinder from a vehicle with caliper pistons of the same diameter. you also need to ensure all the pedal linkage is free, the linkage joints and pivots are not worn out and the pedal has some sort of positive pedal return spring so the master can be fully retracted. another overlooked item is the pushrod free play in the booster to master cylinder connection, as per the link that miracle posted.
drum brakes have return springs on the shoes to return the wheel cylinder pistons. there is a star wheel adjuster and a park brake adjuster that needs to be kept within certain limits for the shoes to be kept close to , but not dragging against, the drums. disc brakes don't have return springs. they have a square O ring that rolls with the piston as the brakes are applied. when the brakes release the O ring rolls back to be seated in it's groove and brings the piston back into the caliper at the same time.the piston is allowed to slip on the O ring to compensate for brake wear so adjustment is never needed. obviously with either of these systems, the master cylinder pushrod needs to be adjusted so the piston in the master cylinder comes all the way back to allow fluid to return to the reservoir. drum brakes have a residual valve built in so that there is always a slight pressure in the system. this is because the cups in the wheel cylinders need to stay pressed against the cylinder walls so air can't get past them and into the system. there is usually about 10 psi in the drum brake system. a disc brake system usually has no residual valves because the master is above the wheels. when the master is on the frame then a residual valve, usually about 2 psi, is used to keep the fluid from syphoning back to the master cylinder and bringing the caliper piston along for the ride.
make sense?

[dsraven]

a factory master cylinder is set up for drum/drum brakes and also is a single system. a single system means there is only one circuit in the brakes and it applies ALL the hydraulic brakes from a single piston in the master cylinder. these master cylinders only have a single brake line coming out of them and will likely have a residual valve built in at the outlet. a dual system has 2 seperate brake systems in a single master cylinder, a front and a rear, so if you happen to blow a brake line or some other issue happens in either the front or rear brake circuit, then you will still have the other circuit to stop the vehicle. manufacturers started installing the dual system brakes waaay back for safety reasons. it would really make sense to install a dual circuit master cylinder in your trucks, one that is sized according to your needs.

[jweb]

It sounds like you may have multiple problems.

You need a combination valve and residual valves. This shows how different systems should be plumbed
http://www.mbmbrakeboosters.com/inde...id=8&Itemid=16

Brake pedal ratio
https://techtalk.mpbrakes.com/how-to...ng-pedal-ratio

Also, do you have a booster or just master cylinder?
If you just have a master the bore size should be 1" or less.

[_Ogre]

i run a disk/disk 03 mustang hydroboost under floor and cut off 3" of the pedal
initially the brakes were bled with the cab off and 1.5" down was locked
this worked fine for the first 8 years, until i had a brake issue
replaced the mc this spring and couldn't get the brakes to bleed
ended up replacing rear calipers and going from oem mustang differential valve to single line
nothing really helped until it dawned on me that i had cut the pedal
removed pedal and bled from below pulling on the pedal arm
it worked, i have 90% pedal that i originally had, still a little more stroke but brakes work great
i believe there was some air in the mc from not bleeding with full stroke

[daveshilling]

Originally Posted by _Ogre

i run a disk/disk 03 mustang hydroboost under floor and cut off 3" of the pedal
initially the brakes were bled with the cab off and 1.5" down was locked
this worked fine for the first 8 years, until i had a brake issue
replaced the mc this spring and couldn't get the brakes to bleed
ended up replacing rear calipers and going from oem mustang differential valve to single line
nothing really helped until it dawned on me that i had cut the pedal
removed pedal and bled from below pulling on the pedal arm
it worked, i have 90% pedal that i originally had, still a little more stroke but brakes work great
i believe there was some air in the mc from not bleeding with full stroke

YES, that makes complete sense with a cut pedal not necessarily bleeding the brakes correctly. Definitely something to try!

[daveshilling]

Originally Posted by Foot Stomper

Well then, I guess frankenstein is your issue then. Running discs with a factory MC is a big mistake.
Cheers

Edited the original post to explain that one truck is full custom, and the other will be. No factory anything except the frame rails and sheet metal.

[dsraven]

jeez ogre, you didn't have to pull your cab off just to show your system.....lol.
nice explanation and pic showing your hydroboost set up.
it makes sense to check the master cylinder travel when setting up the pedal, a full pedal stroke should equal a full master cylinder stroke with just a little bit of pedal stroke left over for safety. then the pedal could be shortened if it needs to be, like ogre did, once you find the sweet spot where the brakes are full on and hard pedal. personally i wouldn't shorten the pedal unless you have legs 9 ft long so it is uncomfortable or it is just simply waaay to high off the floor to find it in a panic stop situation. possibly adjust some of the linkage to bring the pedal height down? make an adjustable pedal where it connects to the lever under the floor?

[whitedog76]

That's thinking with your dipstick OGRE! I just mocked up a hydroboost setup on my 54. I did notice how the hydroboost plunger travels about 1/2 the distance as a vacuum boost setup. I'm going to keep that in mind for down the road.