The Stovebolt Site Forums The Shop Area Driveline 1948 Truck - Brake Fluid Question

Need help, any tips or procedure for how to properly fill brake fluid on a new, dry system. I have a 1948 1/2 Ton Truck just installed new master cylinder, 4 new wheel cylinders, new brake lines, etc. The whole setup is dry - Is there any particular procedure to start adding the fluid, to avoid any air bubbles that enter into the lines?

Did you bench bleed the M/C before you installed it ? If not it will take a bit longer to get all the air out. One way, probably not the best, is to open all the bleeders at the same time and let the system gravity bleed itself. Make sure you keep the M/C full while the bleeders do their thing. Once you see fluid coming from a bleeder close it off. Keep watching until each is closed. The other way is to use an assistant to pump the brakes while you crank the bleeders open one at a time, starting with the RR. The assistant pumps the peddle until he has some pedal then you open the bleeder and the pedal goes to the floor. He holds it there while you close the bleeder. Repeat until the fluid runs w/o air bubbles. When the fluid runs with no bubbles close the bleeder again and move to the LR, then the RF and finally the LF. Others will respond with their way.
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Some people recommend bench bleeding the master cylinder before attaching to the truck. I have done it your way and also bench bleeding and both ways were successful. So start adding DOT 3 or 4 to your master cylinder and as you bleed be sure you don't let it run dry or you will introduce air into the system. Start with the wheel cylinder farthest from the master cylinder which should be the left (driver's side) rear, the right rear, left front and finally right front. Look for leaks in your new system.

It is easiest to have a helper to push on the brake pedal as you bleed but you can do it yourself with those commercially available vacuum systems or simply putting a long hose on the bleeder screw into a container of fluid so air doesn't suck into the cylinder when you release the pedal (when working alone).

If you don't have tight pedal action when finished consider adjusting the shoes tighter before proceeding with more bleeding or quitting in frustration.

Kent

Bobby, I'll throw another wrinkle into your project....consider Dot 5 brake fluid. Due to the complete rebuild you've just completed that's an option that's only available with a complete rebuild.

Had I used Dot 5 several years ago on one of my cars, I wouldn't have had to redo the job 4,000 miles later (but 20 years later).

Lots of opinions on this subject, just do some research.

Ron

Use a Mityvac hand pump vacuum bleeder and a set of speed bleeders with a spring-loaded check valve to prevent air from getting sucked back into the wheel cylinders. Harbor freight has a Mityvac knockoff pump that works pretty well. Be sure to keep the master cylinder reservoir full to avoid getting more air into the system.

If you choose to use a helper on the pedal, DO NOT pump the brake. Push the pedal a time or two and hold it down, then open a bleeder. Repeated pumps on the pedal breaks the air up into tiny bubbles that are very hard to get rid of.
Jerry

Competent brake pedal operating helpers are hard to find. Choose one carefully. NEVER ask your wife!

On both my 38 and my 51 I used the method shown in the shop manual. I used a tube from the bleeder to a bottle and did all the bleeding myself. I started with the furthest away and I did not bench bleed the master. Many years ago with my buddies we would have someone "pump up the pedal" and hold while bleeding. I found this to be unnecessary. These are both stock systems that I am talking about.

Many thanks all, really appreciate the tips, keep them coming. I'm sensing it will go much better with 2 people on this one. Curious about the DOT5 brake fluid, is it heavier, thus less chance for air or?

Bobby, you'll find a cut/paste below that's a bit wordy but covers most of what I've read over the last three or four decades on Dot 5 vs Dot 3/4 fluid. Dot 5 is a silicon fluid, it doesn't absorb moisture like glycol based brake fluids. As mentioned below if you drive a vehicle often or change brake fluid every couple years Dot 3 or 4 is great (just don't get any on your paint).

The reason I switched to 5; Due to life my 70 340 Duster went for about 10 years without being driven and was pretty much forgotten and neglected in the back of the shop. When I started bringing it back last year I had to rebuild/replace the complete brake system due to internal corrosion. For those that don't know, KelsyHayes 4 piston calipers are a bit of a pain. Would Dot 5 have prevented this......I think it may have. However had any moisture gotten into the system I'd still have had rust at the low points where it would have settled.

Cut/Paste from Steel Soldiers.com

First of all, it's important to understand the chemical nature of brake fluid. DOT 3 brake fluids are mixtures of glycols and glycol ethers. DOT 4 contains borate esters in addition to what is contained in DOT 3. These brake fluids are somewhat similar to automotive anti-freeze (ethylene glycol) and are not, a petroleum fluid. DOT 5 is silicone chemistry.
Brake system materials must be compatible with the system fluid. Compatibility is determined by chemistry, and no amount of advertising, wishful thinking or rationalizing can change the science of chemical compatibility. Both DOT 3-4 and DOT 5 fluids are compatible with most brake system materials except in the case some silicone rubber external components such as caliper piston boots, which are attacked by silicon fluids and greases.
The big bugaboo with DOT 3-4 fluids always cited by silicone fluid advocates is water absorption. DOT 3-4 glycol based fluids, just like ethylene glycol antifreezes, are readily miscible with water. Long term brake system water content tends to reach a maximum of about 3%, which is readily handled by the corrosion inhibitors in the brake fluid formulation. Since the inhibitors are gradually depleted as they do their job, glycol brake fluid, just like anti-freeze, needs to be changed periodically. Follow BMW's recommendations. DOT 5 fluids, not being water miscible, must rely on the silicone (with some corrosion inhibitors) as a barrier film to control corrosion. Water is not absorbed by silicone as in the case of DOT 3-4 fluids, and will remain as a separate globule sinking to the lowest point in the brake system, since it is more dense.
Fluid boiling point DOT 4 glycol based fluid has a higher boiling point (446F) than DOT 3 (401F), and both fluids will exhibit a reduced boiling point as water content increases. DOT 5 in its pure state offers a higher boiling point (500F) however if water got into the system, and a big globule found its way into a caliper, the water would start to boil at 212F causing a vapor lock condition . By contrast, DOT 3 fluid with 3% water content would still exhibit a boiling point of 300F. Silicone fluids also exhibit a 3 times greater propensity to dissolve air and other gasses which can lead to a "spongy pedal" and reduced braking at high altitudes.
DOT 3 and DOT 4 fluids are mutually compatible, the major disadvantage of such a mix being a lowered boiling point. In an emergency, it'll do. Silicone fluid will not mix, but will float on top. From a lubricity standpoint, neither fluids are outstanding, though silicones will exhibit a more stable viscosity index in extreme temperatures, which is why the US Army likes silicone fluids. Since few of us ride at temperatures very much below freezing, let alone at 40 below zero, silicone's low temperature advantage won't be apparent. Neither fluids will reduce stopping distances.
With the advent of ABS systems, the limitations of existing brake fluids have been recognized and the brake fluid manufacturers have been working on formulations with enhanced properties. However, the chosen direction has not been silicone. The only major user of silicone is the US Army. It has recently asked the SAE about a procedure for converting from silicon back to DOT 3-4. If they ever decide to switch, silicone brake fluid will go the way of leaded gas.
Brake system contamination
The single most common brake system failure caused by a contaminant is swelling of the rubber components (piston seals etc.) due to the introduction of petroleum based products (motor oil, power steering fluid, mineral oil etc.) A small amount is enough to do major damage. Flushing with mineral spirits is enough to cause a complete system failure in a short time. I suspect this is what has happened when some BMW owners changed to DOT 5 (and then assumed that silicone caused the problem). Flushing with alcohol also causes problems. BMW brake systems should be flushed only with DOT 3 or 4.
If silicone is introduced into an older brake system, the silicone will latch unto the sludge generated by gradual component deterioration and create a gelatin like goop which will attract more crud and eventually plug up metering orifices or cause pistons to stick. If you have already changed to DOT 5, don't compound your initial mistake and change back. Silicone is very tenacious stuff and you will never get it all out of your system. Just change the fluid regularly. For those who race using silicone fluid, I recommend that you crack the bleed screws before each racing session to insure that there is no water in the calipers.
Since DOT 4 fluids were developed, it was recognized that borate ester based fluids offered the potential for boiling points beyond the 446F requirement, thus came the Super DOT 4 fluids - some covered by the DOT 5.1 designation - which exhibit a minimum dry boiling point of 500F (same as silicone, but different chemistry).
Additionally, a new fluid type based on silicon ester chemistry (not the same as silicon) has been developed that exhibits a minimum dry boiling point of 590F. It is miscible with DOT 3-4 fluids but has yet to see commercial usage.
By Steve Wall

Now all the above said, likely the smartest thing is to just change the brake fluid every two or three years. I actually do this on my 3500 95 Dodge Diesel and the fluid always comes out fairly clean. Every fall it pulls a 12,000 lb. hunting trailer across the Rockies and I Don't want to even think about a brake failure, hence the extra attention to details.

Good Luck, Ron

Not much to add, but I can remember my dad, being an Army aircraft mechanic, always pulled me into his brake repair or replacements on his cars. I probably was 11 or 12 years old. I was the all important pedal pusher! Down, release, down, release and I took my job very seriously. I still bleed my brakes the same way.

Something I read concerning our older pickups is that the master cylinder cap has a small orifice to allow air in or out to relieve pressure/vacuum. This constant small change of air is one way moisture is allowed in the system which makes it important to change the fluid every couple of years. Newer vehicles have a diaphragm in the cap to alleviate the pressure changes without changing the air. My wife is a very good brake bleeder assistant!
One other thing, Kent mentioned checking your brake shoe adjustment if you cannot get a good pedal; this happened to me last week. I had already finished with the wheels and had adjusted the shoes to what I thought was a slight drag before I bled the system. After a test drive which showed poor brakes I was about to try rebleeding when I noticed that one wheel seemed out of adjustment. It turned out all four needed a good bit of tightening and the latest test drive went well.
Lee

Bruce, as a youngster I did the same many times. "Push, release, push, release."

I like the speed bleeders that Jerry suggested. Very easy, no need for a helper, and no chance for air.

Many thanks!!!

I've heard of speed bleeders but don't know anything about them. Can you buy them at NAPA or the like? If I went in to buy some for my '50 would I need a part number or would they know what to sell me? Seems like they're the way to go when you're working on your brake system and wanting to update.

Great information, especially for a newbie like me. I did find a size chart for the Speed Bleeders. Size Chart for Speed Bleeders