Hot Rods Help me troubleshoot poor brakes

1940 Lincoln

**More info on post 17 - no resolution**

Trying to correct someone's attempt at building a hotrod.

Aftermarket power brakes (cheapest catalog find, my guess) l. Pedal travel inside is 6 1/4". Using original pedal setup with extension.

Brakes won't lock up and merely slow as a suggestion.

Funny it ties into my screen name from 20+ years ago.

 

For all the "experts that reinvent the wheel" with brakes.
Simply restore the brakes to stock specs [maybe with a tandem m/c of identical bore]

If boosted brakes are a "must" use an inline PBR VH44 booster which can be mounted anywhere [VH24 for drum/drum]

 

Not an option here. Nothing stock anymore.

 

Typically, the ratio of pedal travel to master cylinder travel for old cars with non power brakes is about 6:1. You need to be about 4:1.

See this page. https://help.summitracing.com/knowl...or power brake systems, the,A 4:1 pedal ratio

 

There is a shitload of math involved.
And most brake threads here are to justify the clusterfuck they've already made .
Sometimes you need to start from scratch and bin half the expensive mismatched componentry.

Try and find a common denominator with your brakes [eg matching front and rear brakes] and copy the vehicle that it came from
The manufacturers have done all the calculations for you.

eg: It you have "G-Body" discs and calipers , then try and match the rear drums, shoes and wheel cylinders. [this gets the bias close enough]
Then look at the G-Body combination valve and similar M/C diameter booster size and pedal ratio.
[You can alter pedal ratios with bore size "fluid mechanics" to get the same results, but this requires maths]

Under floor M/C's require the addition of residual valves to prevent syphoning back [pad retraction]

If a person can get their Hotrod to stop as good as a mid 80's Honda they will be miles ahead of most of the hackjobs out there.

 

A suggestion if you don’t mind a partial start over. Remove the vacuum assist canister, replace the rod, and try the system as a non power. If you know the ID of your MC great, if not find out now.

Even with a power assist MC your brakes should work but have a hard pedal because the MC that’s there is too large.

If it were me and I found it that way I would install a 7/8” bore MC and see how you like it. (Wilwood has them) If needed, a Midland vacuum assist canister on an inner fender well to the front disc brake line on the MC.

If your cars rear end has been upgraded to more modern drum system which came from a vehicle with power brakes you also need to replace the wheel cylinders with a smaller diameter.

I speak with experience on converting to what works on a mid 50’s Ford conversion to disc brake front and later larger drum brake rear. The only difference is a firewall pedal. Good luck

 

Idk what your solution is but I installed one of those cheap units in my car and I never could get it to work properly. I cut up the pedal assembly and modified it for correct pedal ratio and it still wouldn’t work right. I finally gave up and installed a non powered hanging unit which works great. I would prefer a floor pedal and may revisit it in the future but I wouldn’t know which assembly to buy. They all look the same to me.

 

Before you re-invent the wheel, to me "spongy" usually means air in the lines.

my 2 cents

 

I've bled a pint through the system with no air. That was my first thought, then I double checked the rear drum adjustment and front calipers.
I hoped it was a simple solution like a bleed.

 

@nobrakesneeded (cute!) please provide as much information as possible.
As @Kerrynzl said, most of these threads end up like a battlefield, with big holes in the ground and no safe place to walk.
Front spindle, rotor, caliper including size.
Master cylinder, brake pedal and booster including size.
Rear axle, drum, wheel cylinder, and shoe sizes.
Any other components in the system.

 

And getting information requires "waterboarding"
So far all we know is "he didn't build it"

What collection of parts is this made from.

 

and it's a little over 6 inches, he says!

 

Everything above, plus:
What is the master cyl ?
Bore size ?
What calipers ?

 

I'm back onto this project after focusing on a few other issues.

Here's where I'm at on it.

1" bore generic master cylinder and booster installed under the floor board with residual valves.
GM5474 calipers up front. P/N references 69 and newer full size cars
Ford 9" rear end of unknown vintage.

Stopping power is minimal at best. Won't lock up on a panic stop.

Soft upper pedal travel but it firms up at bottom of travel and doesn't lay on the floor.

Front brakes aren't grabbing like they should. I can turn the front wheels by hand with the brake pedal pressed.

 

As others have said,
Pedal ratio, master cylinder and calipers all need to match/work together.

For example, my wagon has GM calipers so I found a master cylinder with a bore diameter that matches the bore of the vehicle my calipers would have come on.

I measured the pedal ratio on my mustang that came with power disk brakes and replicated that ratio on the wagon when adding power disk brakes.

Finally, most kits come with an undersized booster. My frost booster was too small and I had to source a much larger one that would fit, and make sure you have sufficient vacuum to work the booster.

a few other things,
Make sure all parts are actually working. A seized caliper or booster will make the brakes not work, as will bad flex hoses. These things have more problems due to age and sitting than miles so his because they were new and not driven doesn’t mean they are any good.
And you may need a pressure bleeder to get all the air out on a custom system.

 

I'll bring this back to the top
GM5474 front calipers reference to Cadillacs, Buicks and Oldsmobiles of the 70s.
Ford 9" rear of unknown vintage
Was 1" bore master cylinder with booster (corvette style)
5:1 pedal ratio
As noted before - very soft upper pedal, hard pedal toward floorboard - pedal does not go to floor

Upgraded to 1 1/8" bore master cylinder after discussion with reputable manufacturer
Added proportioning valve for rear drums
proper residual valves in place as master is installed under the floorboard
The stroke of the master is 1 3/16" to bottoming out - the pedal won't push past 3/4", there is no binding in the pushrod and the pedal is still 2 1/2" from the floorboard, so the pedal is not bottomed out.
Equal sized reservoir fluid chambers - plumbed front to front and rear to rear.

Changing the master and adding the proportioning valve made no improvement in the brakes.

I'll be looking at the front calipers next.

What am I missing?

 

Username does not check out.

This is a shot in the dark, but a faulty brake booster vacuum check valve can cause all sorts of problems. For $7.99 and a few expletives to install it might be worth a shot.

 

Are you confident fluid is moving through the lines freely?

 

The booster. The last two boosters I’ve bought have been no good out of the box. One for a Vette one for a Mercury. Both reputable manufacturers, swapping them for another unit cured the issue, the Vettes would drag the brakes the Merc gave no assistance so required a huge amount of pedal pressure

 

Yes. Good movement and no air.

 

The problem is insufficient line pressure, although poor quality pads and / gazing of pads (due to repeated use with insufficient pressure) shouldn't be ignored. Booster size hasn't been mentioned. Bigger is better. 7" single is rather ineffectual. 8" dual starts to get things stopping. 1 1/8" master is too big unless the booster is very powerful.

Chris

 

Just for shits and giggles have you adjusted the rear brakes so they have a slight drag ?

 

Friction materials have changed drastically over the last 25+ years. I've wondered how much that may have affected brake pedal feel compared to earlier asbestos-based brake linings and pads. Many newer vehicles with metallic or ceramic brakes seem to have a slightly softer pedal, though they seem to stop well.

And it's also not difficult to imagine replacement brake materials being made differently to work well with anti-lock brake systems.

 

Your collection of parts should work together.
You say here's no air in the system but, I suspect that could still be the problem contributing to the spongy pedal. Just because you're not getting any bubbles out of the line when bleeding the brake doesn't mean there's no air trapped in there somewhere. Air will not bleed out if it is trapped in a place where pressure is making it go up hill when the path to the wheel cylinder or caliper is down hill. An example of this is a brake light pressure switch that is pointing up.
Inspect the lines to see if there's any place air could be getting trapped.

 

I also had two bad boosters new out of the box,
One didn’t provide any boost and the other caused the brakes to drag.

I’d replace the booster,

the other thing id try is pulling a caliper offf and making sure the piston moves. Put a 2x4 in it and press the pedal. I’ve had lots of seized calipers

 

Back on the lift with this nightmare.
Reputable hotrod brake company suggested I go up from 1" to 1 1/8" master....I did - no improvement
I changed the front calipers and hoses because what was there looked a bit crusty - no improvement
I checked the function of the booster - 7" dual - all panned out there - a bit small but no room to upsize
Verified proper adjustment of rear brakes - new hardware previously - all checked out
Will now decrease master cylinder size to see of increased pressure will get me stopping brakes - we'll see what happens this weekend

 

How much free play is in the pedal ? If you have an adjustable rod from the pedal to the back of the booster, adjust the free play to a bare minimum and see if that helps.

 

If the pedal feels spongy, it's unlikely to be the booster. A bad booster has a rock hard pedal but doesn't stop. It may even cause the engine to stall when the brakes are applied if it is leaking vacuum.

Pictures of the calipers and mounts may help with diagnosing the problem. Calipers on the wrong side or mounted at a bad angle will cause a spongy pedal.

 

Are you sure your master cylinder is for disk/drum and that the disk side is plumbed to the front brakes the drum side is plumbed to the rear brakes?