Hagerty.com

5 ways to bleed your brakes: Get the air out

Bleeding the brakes is an inevitable part of DIY automotive repairs. Air can enter the brake system during repairs, but the more common reason is far more insidious. Brake fluid is hygroscopic and readily absorbs water from the atmosphere. Water in the brake fluid lowers its boiling point. Heat boils the brake fluid, and the resulting steam leaves air in the lines. Brake fluid is incompressible while air is easily compressed and manifests as a spongy brake pedal or worse.


This is a companion discussion topic for the original entry at https://www.hagerty.com/articles-videos/articles/2019/06/12/5-ways-to-bleed-your-brakes

For some British cars, there are a few more steps.

Some cars according to the manual require for the front drum brakes that the front of the car be elevated. As well, the adjusters on the brakes be adjusted so the wheel cylinders have the adjuster so the wheel cylinder is fully retracted. So air does not get trapped in the cylinders.

I’ve also used special one way brake bleeder fittings that you don’t require to be under the car whilst someone else pumps the pedal. They worked great. I used them on all 4 drum brakes as well as the clutch slave cylinder.

Another British car Oldtimer trick is after bleeding, is to place a board on the pedal over night and have it jammed into the seat front. So there is constant pressure overnight on the pedal. Helps to get a good stiff pedal.

Enjoy bleeding your brakes!!!

Ah yes, I own very rare British army Land Rovers -101 Forward Controls. They are very nasty to bleed. The army used to hang them up with the NATO hitches, at 45 deg

Good info but you left out one item. Always buy the very very best heavy duty brake fluid money can buy. A little extra up front cash can be a game changer if you race or haul a trailer. Jim

Incidentally, I just flushed the brake fluid in my wife’s car last weekend. I have tried many of the reported methods for bleeding the brakes, usually enlisting my wife to help with the two-person method. I have also tried using a vacuum pump to draw the fluid through, but I was never successful in preventing air from leaking in at the threads on the bleeder valve. Then I finally tried a simple, one person method that works great. I connect a clear tube (so I can see bubbles) to the bleeder valve and terminate it in an empty 1 quart bottle. I then locate the bottle at an elevation higher than the bleeder valve. You could add a little fluid to the bottle if you want, but I have found that it is unnecessary, at least when I am flushing the brake fluid, since the bottom will be covered in fluid within a few pumps. I then open the bleeder valve and pump the brake pedal (gently). I have found that I can pump the back brakes 10-15 times or the front brakes 5-8 times before needing to refill the master cylinder. Any air in the tube/hose is quickly purged and I end up with no air in the system. This method has never let me down yet. The two person method probably uses less fluid, if all you are doing is bleeding the brakes, but for a fluid flush, this method works great. Best of all, it requires no expensive equipment.

Speed Bleeders should have been mentioned. One way check valve and pretty cheap considering brake bleeding should be a matter of routine maintenance. I own 20 British cars and attest the rear cylinders tend to be the first to seize due to poor maintenance. As the rear brakes do little than keep the tail from wagging the dog, owners (me) tend to overlook the lose of all rear brakeing…I learned that on my 240Z with alloy rear cylinders PERMANENTLY seized.

1 Like

On the Motive power bleeder, those are a PITA to clean. You can spend less total time by filling the master up to the top, pressurizing it, and then bleeding - just need to keep an eye on the level and go through the whole depressurize/fill/pressurize cycle if you have to.

Using the gravity method works best if you place the catch tank above the wheel cylinder,not below it as show in the image. Placing the catch tank higher than the master cylinder gives the best results as the bubbles will always try to move to the highest location in th system.

The best brake fluid i think you can use is DOT 5 b/c it is silicon and it does not absorb moisture. I’ve had it in my 55 Chevy for the last 9 years, and it looks as clean as the day i put it in!! PLUS if it gets on the paint it does not ruin the paint.

The Silicon fluid might not look dirty but moisture still gets in the system. I have had to rebuild them systems too. Save your money.
I have been telling my club members to suck the fluid out of the master cylinder and add new fluid every year. Much easier than trying to flush the system. The fluid will turn around.
I have much better luck with the pressure bleeder by connecting it to the bleeder screw at the wheel and pushing fluid up to the master cylinder. The 70’s & 80’s GM cars are a pain to get the air out of the rear brake lines and this works. You might get a little air in at the wheel cylinder but after disconnecting the pressure bleeder just let a little fluid gravity bleed.

The best way that I have found is to buy an extra cap for the master cylinder, drill a hole for an air fitting and install it. Then use a air regulator, set it to 5-10 PSI while on the cap, hook up an air supply and pressurize the reservoir and then go to each wheel and open the bleeder with a tube and catch basin. One person job and works great! Only cost is for a reservoir cap.

“I have been telling my club members to suck the fluid out of the master cylinder and add new fluid every year. Much easier than trying to flush the system” OMG, you are pushing any decomposed brake fluid, damaged or partially dissolved seals etc back up into the system to be distributed elsewhere. Sounds like a TERRIBLE idea to me. The calipers get VERY hot, boil the fluid, causing it to break down and turn to carbon and sludgey goo, leaving all sorts of nasty stuff. Bleeding is intended to remove this crap, not force it back up into the MC! This idea sound positively disastrous to me.

Hmm I been in the auto repair business for 50 years, it works. If you got seals coming apart flushing won’t fix it either.