Technical Ford juice brakes - Problem, help needed!

Hi Guys,
Wondering if any of you can help me with a problem I am experiencing on my 35 pickup.
I have fitted up all of the brakes, 1940 ford hydraulic brakes all round with a mustang dual pot master (no booster). I have bled the brakes and adjusted them but when I take it for a drive I have to pump the pedal at times to be able to brake. This does not happen all of the time, only when you haven't pushed the brake pedal for a couple of minutes or so. It's like the fluid is being pushed back and needs a couple of pumps of the pedal to get it back to the cylinder to be able to push on the piston to engage the brake.
I know most people would say there must still be air in the lines, but I have bled them now numerous times and I still have the same issue.
Can anyone give any help, tips, advice?

 

If the master doesn't have RPVs you will have to add them, 10# in each line.

 

I had a similar situation... my new master cylinder was junk. Also, did you replace the wheel cylinders? If you did, did you resist squeezing the wheel cylinder pistons with your fingers? I know it's fun to do, but it can ruin the seal in them. Other than that, I might try blocking off the fronts, then the rears to see if I could isolate the problem.

Also, it can take a while to get all the air out. Before I did anything else I'd bleed them a few more times. Are you still getting air at the bleeder?

 

and there's that too.

 

Not true, the purpose of an RPV is to keep slight pressure on the wc seals to prevent air from leaking in around them while the brakes are not being used. Once the system is fully bled, it's sealed. An RPV has nothing to do with keeping the pedal firm, 2lb, 10lb, whatever, is not nearly enough to overcome the spring tension on the shoes.

Sounds like (A)your brakes need to be adjusted, (B) your pedal rod needs to be adjusted, or (C) as stated above, your MC is defective.

 

Did you bench bleed your m/cyl first.? The m/cyl is quite low in the system so you may need the valves. I have a 35 p/up too and would like to fit a similar system, keep Intouch let me know how you went and what you did..
Rob

 

Well, your explanation of residuals is different than most, but also totally wrong, as most. The purpose of drum brake residuals is to prevent air from entering past the wheel cylinder cups during fast brake release, caused by a partial vacuum because of the strong shoe return springs. The benefit is a slightly firmer pedal.

 

The MC is at or below the wheel cylinders on these under floor applications. I ran in to the same problem on the pickup when I split the braking system. It is necessary to install residual valves if they are not in the MC. 10 PSI for drums and 2PSI for disc. Bench bleeding of the master is a must.

Talkwrench: How come your FB avatar dog is not in the cab of that pickup?? BTW, Perth was very interesting for the day I was there while on holiday.

 

V8 Bob, the shoe return springs are putting pressure on the pistons/cups in the wheel cylinders. How will that create a vacuum in the wheel cylinders?

 

Not to muck up this guy's post, but please explain how one would create a vacuum in fluid...?

 

That's funny Andy. Anyway, these will help bleed the system. These are real juice brake bleeders. Put a hose on the*******, run it to a can of fluid and pump the***** out of them. Get them from Roy Nacawicz, (734) 654-9450



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The return springs can move the shoes/pistons fast enough to pull air (a partial vacuum) past the cups without residual (checked) pressure, or metal cup expanders (re-enforces the cup lip seal) that replaced residuals by the mid '70s in most applications.

 

interesting... , wouldn't it actually slightly pressurize the system until the excess fluid drained back into the MC reservior?

That's one way to do it Scott!

 

I guess I'm not seeing it. In order to create a vacuum in the system, the springs would have to be pulling the cups AWAY from the fluid, not pushing them towards it.

 

The wheel cylinder seals can allow pressure from outside without some pressure exerted inside, by their design. A small residual pressure or cup expander prevents this. Any pressure that passes the seal from the outside would be..............air, correct?

 

Yes, I totally understand that. I just can't grasp the idea of how brake shoe springs pushing the pistons back into the cylinders quickly can create a vacuum. The fact is that the action which is pushing fluid back into the master cylinder is creating pressure, not a vacuum.

Once everything is back into an "idle" position, I can see where some air might get past the cups, but it certainly doesn't happen because of the shoe springs, which I believe was stated in your first post that I quoted.

 

Ebbspeed, When the brakes are applied the pressure from the pedal pushes on the fluid and the fluid applies pressure to the piston cup seal eliminating a leak. When the brakes are released there is no pressure on the fluid to maintain the seal and the pistons are returned to rest by the spring tension. The brake spring return cannot build up pressure against the seal as there is no resistance in the fluid return to the master cylinder. Thus the use of a RV. Dirty fuel says he has correctly bled the system and has brakes after a couple pumps, this to me would more likely indicate that the master cylinder displacement is to small for the system. What size is the master cylinder or what model car was it out of?

 

Thank you*****.

 

If the problem is that you are*****ing air back into the wheels cylinders then you would have a low and spongy pedal, and if you then go to each w/c and crack the bleeder, you will see air bubbles come out.
If what you are experiencing is low but firm pedal that comes "up" as you pump the brakes and remains firm untill the next time you go to hit the brakes, then you either have the wrong size master cylinder bore or your brakes need adjusted properly.

 

Dick and V8 Bob, the statement was "The return springs can move the shoes/pistons fast enough to pull air (a partial vacuum) past the cups without residual (checked) pressure, or metal cup expanders (re-enforces the cup lip seal) that replaced residuals by the mid '70s in most applications." The fact that the spring ARE pushing the pistons is creating pressure to both overcome the pressure rating of the residual valve, plus whatever additional slight pressure is required to PUSH the fluid back into the MC. I understand the purpose of the RP valves. What I do not understand and do not think is correct, is the suggestion that, when the shoe return springs are rapidly pushing the pistons and cups (and thereby the fluid as well), there is a VACUUM created within the system. System at rest, whole different story, but during that brief period of time when the WC pistons/cups are retracting due to spring pressure, there is no vacuum in the wheel cylinder.

 

Ebbspeed, If I understand this correctly this is an air ingestion issue not really a vacuum. This is more of a concern with worn or contaminated wheel cylinders than new ones. Upon release of the pedal pressure the spring pressure acts to return the brake shoes to rest position, at this time the tension of the spings is rapidly contracting the wheel cylinder piston and this produces a low pressure area at the piston. This low pressure area is not sufficient enough to get a good seal on the piston seal cups and dependent on the quality of the seals a very small amount air can actually get drawn into the system. In most cases this is unlikely with new parts unless they have been****embled incorrectly. You usually dont see this with a disc brake unit as there are no return springs and the rotor run out is used to position the pad at rest. RV on a disc unit keeps the pads closer to the rotor if there is a slight runout of the blade and decrease pedal travel during braking .

 

So, I too will be running a mustang dual reservoir on a 40 with drums all around... what's the consensus, Residual valves or no?

 

Dan, I run them using the 53-6 fronts and 8 inch maverick. What can it hurt. I'm running a 7/8 ths master.

 

This is a constant conversation on this board. Mix and match is the problem here. It's just plain the Wrong Master Cyl. 40 Fords used 1/4" steel brake lines. All mustangs used 3/16 steel lines. You can not displace enough fluid from a Mustang master to charge 1/4" lines and make the wheel cyl move enough to apply the brakes, that's why 2 pumps will do it instead of 1 push. Even if you plumbed 3/16" lines on the chassis the 40 wheel cyl. still needs more volume than the Mustang can move in 1 pump. Residual valve issue aside your running an inefficient master for the wheel cyl.
The Wizzard

 

Are you loosing fluid? Do you have top off the M/C? It sounds like a classic leak problem to me. I used a 1" bore Camero M/C for years with no problems. No RV's. 40 brakes in front and Maverick in the back. The Camero M/C is the same bore as the 40 Ford.

 

So two pumps and call it a day? This is the set up I've read a million times for adding 40 juice to model A's.

 

I installed a dual master in my Willys in the stock location. First reman master was bad, never bled up. Bought a NOS one on ebay, bench bled the hell out of it. Then proceeded to crack every line and bleed at each joint starting at the master working out. That's about a year ago and still fine. Here's a good explaination from "MuscleCar" Scroll towards the end of the episode.

http://www.powerblocktv.com/episodes/MC2013-02/oldsmobile-restomod-returns

Good luck.

 

I reverse bled my system, don't have to bench bleed. Worked out cool. However I am having the same two pump issue. Pedal doesn't go to the floor, but needs the second pump to be solid. I'm gonna readjust all wheels again, and check again for air in the lines.

 

Good point. I have to plumb it all from scratch anyway. Might as well include them rather than retrofitting later.

 

You are suggesting that you can compress a liquid. The line diameter has nothing to do with it, once they are free of air. The M/C will put X amount into the lines, and that same X amount will be distributed to the wheel cylinders. If you have a container that is full of water to the brim, and you pour 1 cup of water into it, then 1 cup of water will spill over the top.