Thru-frame brake line fitting question...

[My52Chebby]

I remember reading somewhere that a braking problem arose (sponginess) from one of these thru-frame fittings. The problem was that the "through hole" was too big, it should be the same inside diameter as the brake line it's attached to.

Now here's my dilemma, there are basically three companies that manufacture these thru-frame brake fittings; Russell, SSBC and Billet Specialties (I think there are also some made by Jegs). NONE of these companies actually tell you what the inside diameter (ID) of the fitting is.

Has anyone on here used these? I'm looking for 03-AN at one end and 1/8" pipe thread at the other end but I'd like the ID to match a 1/4" brake line's ID. I'd like first hand info from someone who has installed them or at least where to find info on the ID of the fittings.

My52Chebby

[kb426]

I've seen the ad in R&C or Street Rodder but can't remember the brand. If my memory returns, I'll post the brand name.

[Bruce Dorsi]

I remember reading somewhere that a braking problem arose (sponginess) from one of these thru-frame fittings. The problem was that the "through hole" was too big, it should be the same inside diameter as the brake line it's attached to.
My52Chebby

I would like to get some clarification on this subject, so I welcome any input from the brain-trust of the RRT participants.  

Why would a larger bore create a spongy pedal?

If the lines are air-free, and filled with fluid, the fluid will not compress causing "sponginess."

I have heard there are issues when silicone brake fluid is used, so possibly this is the problem, not the size of the bore through the frame fittings.

[unklian]

Why would a larger bore create a spongy pedal?

Could it trap a small air bubble ?

[enjenjo]

Why would a larger bore create a spongy pedal?

Could it trap a small air bubble ?

A piece of polyethylene tubing inside the fitting that fit snugly in the fitting, with an ID about the same as the brake line should cure that.

[My52Chebby]

A piece of polyethylene tubing inside the fitting that fit snugly in the fitting, with an ID about the same as the brake line should cure that.

I'll keep that in mind if I can't get info before ordering.

Thanks

My52Chebby

[jaybee]

Could it trap a small air bubble ?

That's my understanding, that the inside of the fitting can hold an air bubble which can't be bled out.

[grazza]

I've also heard of blokes putting a short length of steel brake tube inside the fitting. The tube compresses a bit as the fittings are tightened and that stops the air issue in the fitting. Hadn't heard of using the polyethylene tubing before.

[40_Tudor]

I used them on the front of my 40. I think they are Russell from Jeg's. No brake problems after 12K. I think if you get all of the air out they will work fine. I always gravity bleed brakes.

[My52Chebby]

Why would a larger bore create a spongy pedal?

Could it trap a small air bubble ?

My theory is that if hydraulic brakes work by pushing brake fluid from a big cylinder (the master cylinder) into a small tube (the brake line), you'll increase the pressure in that small tube.

Now... If that small tube has to then fill a bigger tube (the Thru-frame fitting) wouldn't there be a noticeable pressure differential?

BTW, I did purchase and receive my Thru-frame fittings from Billet Specialties and yes, they do have a larger diameter through-hole than the brake line inner diameter.

Billet Specialties' through-hole is nearly 3/8" whereas the brake line is a touch under 1/8". That's a 200 percent bigger area to push brake fluid through.

I've decided to compensate (close in the inner diameter) with a piece of tubing as was suggested.

My52Chebby

[enjenjo]

My theory is that if hydraulic brakes work by pushing brake fluid from a big cylinder (the master cylinder) into a small tube (the brake line), you'll increase the pressure in that small tube.

Now... If that small tube has to then fill a bigger tube (the Thru-frame fitting) wouldn't there be a noticeable pressure differential?

You are confusing pressure and flow. A restriction can effect flow, but not pressure. In other words it can delay the equalizing of pressure in the system, but the pressure across the system will be the same after you have enough flow, and on a brake system, the time is measured in milliseconds.

The total volume change in a brake system is under 1cc, so the flow is not really an issue. But if you have a 1cc air bubble in the system, it can compress and make the system ineffective. The energy is expended compressing the air, rather than moving the slave cylinders. You can see this in a clear line, when the pressure is applied, the bubble gets smaller.

I once worked on a hydraulic system that consisted of a 1/8" line 50 ft long, with a rubber bladder on each end. Squeezing one bladder with your hand would expand the other bladder enough to actuate a micro switch at the other end. Total pressure on the system was 3 psi.

[wayne petty]

there is a strange way to bleed brakes...

not by fully depressing the pedal.. but rapidly usually with ones hand.. bumping the pedal just enough to move the master cylinder piston off its stop ring.. but not enough to cover the fill port with the edge of the piston seal.. this builds pressure but does not create suction when the pedal comes up..

this ratt a tatt a tatt on the pedal works great for me..  

just thought i would share it.

the ability of the large bore fitting to trap air would be an issue.  getting all that air out might be trouble..

perhaps this is a better reason to visit edelbrock to see if they can whip out those russel thru frame fittings using AN3 fittings on both ends. instead of a pipe fitting ..    or perhaps i should contact ARP to see if they could create them.. or the blanks to be machined on their bolt heading line..