Sometimes things just don’t go to plan and even simple jobs can turn into a right pain in the jacksie. Not to worry, we’ll get it figured out very shortly.
Brakes won’t bleed? Your brakes may not bleed for several reasons, but the top five most common issues are:
- Incorrect bleed procedure
- Bleed screw fault
- Flex hose fault
- Caliper fault
- Caliper incorrectly fitted
In this post we’ll cover each of the top five in greater detail and what you can do to fix them. We’ll also have a look at a few other possible causes too.
Warning, some cars are fitted with dynamic braking systems, these systems can be dangerous to work on without disabling them first. Check if your car is fitted with a brake assist system.
1 Incorrect Bleed Procedure
Bleeding the brakes is a pretty simple job, but obviously, it needs to be right. Air in the brake lines will cause your brake pedal to feel soft and your brakes will be dangerous.
But the brake bleeding procedure is easy to get wrong, and I totally understand, after all, it’s not a job a DIYer does very often.
There are a few common methods used when performing a brake bleed service.
- Pump release method
- Gravity bleeding method
- One man brake bleeding
- Brake bleeding tools
Pump and Release
This is the method I was introduced to as a kid. I was the pump man for my father when my feet could barely reach the pedal.
This method requires two people, a brakeman to build a brake system pedal pressure and a released man, who opens and closes the bleed screws.
This method is still popular today in older repair shops and with DIY’ers. It’s a simple three-step process that’s repeated as many times as is needed to get all the air out.
Things you can check:
- Brake fluid level
- Brake bleeding technique
Ensure you have enough brake fluid in the reservoir. If the fluid runs dry, or low, it runs the risk of dragging air or grit into the brake master cylinder.
The correct brake fluid type is very important; the correct type will be marked on your reservoir cap.
Mixing DOT 3, 4, and 5.1 is not advised, but it won’t hurt the system but DOT 5 must never be mixed with any of them. Mixing glycol and synthetic fluids can cause damage to brake system components.
- The brake man builds the brake system pressure by pumping the brake pedal repeatedly. Five to six deep pumps of the brakes and the brake man holds the pedal down and calls out “Hold.”
The brakeman maintains pressure on the pedal until instructed “Pump” by the release man.
Bleeding the brakes on some cars won’t even require removing the wheels, (most cars will however and the wheels sometimes stick to the hub) generally there’s enough access to the bleed screw.
- The release man now fits a ring/box wrench and a clear hose with catch bottle. Clear hose allows you see the fluid run clear and free from air, but it’s not essential.
- The release man opens the bleed screw and fluid rushes through the hose to the catch bottle. A dab of grease around the bleed screw threads before opening to bleed prevents air sneaking in when bleeding.
- The release man closes the bleed screw and calls out to the brake man to start pumping once again.
- Check the fluid level and top up before moving to the next wheel.
This process is repeated on each wheel, two to three times per brake caliper/cylinder until the catch hose is bubble-free.
This is probably the easiest way to bleed brakes. Top up the brake fluid reservoir and simply connect the brake hose and catch bottles to all four brakes at once. Yes, you’ll need four catch bottles ideally to bleed the whole system.
Open the bleed screw and allow the system to self bleed, but you’ll need to check in on the fluid level. This brake bleeding could take some time and you’ll need a dab of grease around the bleeder threads.
Gravity bleed is great for older classic cars where pumping the brake pedal to the floor runs a real risk of damaging the master cylinder seals.
One Man Brake Bleeding
This is a one-man job and the method commonly used to bleed brakes today. A simple bottle 1/4 filled with fresh fluid, it’s important that the bleed hose is below the fluid level in the catch bottle.
Top up the fluid reservoir and connect the hose and open the bleed screw. Now you are free to pump the pedal, three to four times. The old fluid is pumped into the catch bottle together with the air.
The reservoir is topped up before moving to the next caliper/cylinder.
Brake Bleeding Tools
A good brake bleeding tool does make life a lot easier but it isn’t essential. The main dealer will have a very sophisticated electronic bleeding machine that pushes fluid through the fluid reservoir under pressure.
The technician simply opens each brake bleed screw in rotation.
Inexpensive handheld bleeding kits will do the job too. Two types are common one pressurizes the system through the reservoir and the other is a vacuum pump that draws the fluid out through the brake bleed screw.
Both types work well but I prefer the simple one-man vacuum bleed type, which you’ll find listed here on the Brake repair tools page.
2 Brake Bleeding Screw Fault
The brake bleeding screw is a hollow screw that is positioned at the highest point on the exterior of the brake caliper’s fluid chamber or wheel brake cylinder. It is a service screw that’s used to remove air from the brake system.
These little guys live right on the brake caliper, they live in one of the most difficult environments. The heat from the caliper, moisture from the road, and winter road salts cause the bleed screws to become corroded and difficult to remove. And if that wasn’t bad enough, the screws are hollow and easily broken.
It’s not uncommon to damage the bleed screw when removing, causing it to leak and allow air into the brake system.
This is often where a simple brake bleed job goes t**s up. If the bleed screw (aka bleed nipple) shears off or allows air into the system, you can’t drive your car.
Other common problems with bleed screws includes:
- Blockage of the screw bleed passage is common when the dust cap is missing. Moisture and crap gets in and blocks it up solid.
A leaking bleed screw can be caused by corrosion on the tapered seat. The screw seat seals the caliper fluid chamber when tightened. Some bleed screw seats will corrode inside, especially in systems where the brake fluid was neglected for years.
A corroded bleed screw seat will draw air into the system and prevent the system from bleeding.
Things you can check:
- Blockage in the bleed screw
- Leaks around the bleed screw
- Corrosion in the seat of the screw
3 Flex Hose Fault
The brake Flexi hose has an important job, first, it must carry the fluid to the dynamic brake caliper.
The Flexi hoses are flexible for good reason. A car chassis is rigid but the car’s suspension is constantly moving around, the Flexi hoses bridge the gap.
Brake hoses also have to deal with cycling internal brake pressure, it’s a tough environment, add age and contaminated brake fluid and eventually, they break down.
Any restriction in a brake hose will cause problems when bleeding the brakes.
Common problems include:
- Internal walls rotting
- Leaking from fittings
- Clamp corrosion
Let’s take a look at each of these guys and see how they could cause problems when trying to bleed the brakes.
Internal brake hose rotting, happens in older cars, caused by age and contaminated brake fluid. Strips of the internal rubber wall start to peel off and act as a check valve. You may find the brake will work a little bit, but won’t release fully. Many mistakenly replace the caliper.
A blockage in the hose may be caused by badly contaminated fluid. A common cause of contamination is mixing synthetic and mineral brake fluid, easily done and can be expensive to repair.
The inner wall of the hoses collapsing will cause a total blockage.
Checking for a blocked hose:
Using a screwdriver, try to lever the piston back, if it fails to move, open the bleed screw and try again. If it moves now, you have a blocked hose.
Leaking from the fittings is usually around the metal crimp seal. The leak may not necessarily be evident, as the inner wall may be leaking into the outer wall of the brake hose. You may see a bulge in the brake hose when the system is pressured.
A kinked brake hose is easily done, you remove the caliper to fit new brake pads, and the caliper twists, that’s enough to put a kink in the hose and create a restriction to flow which will cause issues bleeding the system.
Brake hose clamps are metal clamps fitted around the rubber brake hose. As the car gets older the clamps corrode and choke the brake hose. This will restrict flow and cause issues braking and bleeding the system.
Things you can check:
- Examine the hoses carefully
- Check for bulges under brake pressure
4 Brake Caliper Fault
The caliper is the business end of the brakes. So far we’ve looked at components that get fluid to the calipers. Now let’s have a look at the main problems with the calipers themselves, they include:
- Seized sliding pins/rails
- Seized pistons
- Seals leaking
- Brake caliper sliding pins
Seized sliding pins are probably the most common brake caliper problem. The modern brake caliper is known as a free-floating caliper. It basically means it’s not fitted rigidly to the hub, instead it floats on sliding pins.
The pins are greased and fitted with protective rubber boots. The boots on older cars break down and allow moisture in, followed by corrosion followed by brake pin seizure.
The pin seizure won’t prevent the brakes from being bled successfully, but the seized pin will cause an unusual brake pedal feel and probably a sticking wheel.
A seized caliper piston will cause the brake pedal to feel horrible and will sometimes cause a creaking noise when the brakes are applied.
A caliper piston seizes or partially binds in the caliper usually because of moisture in the brake fluid.
Brake fluid is hygroscopic, it attracts moisture and the moisture eventually attacks the metal pistons of the caliper turning them rusty and causing them to bind.
Most calipers will have one piston, performance cars will likely have two pots per caliper. A seized caliper like a seized sliding pin won’t prevent you from releasing the fluid from the caliper, but the pedal won’t feel any better after you’ve bled it.
I’ve included both of these faults as they’re common, even though they don’t directly affect the bleeding process.
Checking for a seized caliper:
Using a screwdriver, try to lever the piston back, if it fails to move, open the bleed screw and try again. If it still won’t move, the caliper is seized.
A leaking piston seal will cause a fluid leak and air to enter the brake system. Brake caliper seals leak generally because of wear & tear. Calipers are fitted with rubber seals, an outer seal to prevent dirt from getting onto the cylinder, and a piston seal.
As the piston travels across corrosion on the wall of the cylinder, it damages the seal and a fluid leak develops.
5 Brake Caliper Incorrectly Fitted
If you have removed your calipers for any reason or your fitting new ones, check that they’re on the correct side. Fitting the right-hand caliper on the left side and vice versa is a really easy mistake to make, there’s no shame in it, could happen to a Bishop.
A caliper will likely be marked “L” and “R” for left and right, but another easy way to know which is which – the bleed screw is always at the highest part of the caliper when it’s fitted.
Having the calipers mixed up (left on the right side etc.) will cause the bleed screws to be positioned at the bottom of the caliper. Trapped air will always make its way to the highest part of the caliper and that’s why we always fit a caliper with the bleed screw towards the top.
A faulty master cylinder is also a possible fault, to eliminate it as a suspect, clamp off all four Flexi hoses and pump the brake pedal. If the pedal is firm and holds pressure, the master cylinder is OK.
Having a workshop manual for your vehicle is always a good plan, they only cost a few dollars but will save you a packet. Good manuals cover repair diagrams, wiring diagrams, system operation overview, troubleshooting sections, fastener torque specs, and sequences, etc. all mission-critical info.
Check out all the tools I use here on the “Brake repair tools page.” and if you need brake system parts check out the Amazon link below for cracking deals.Amazon Brake System Parts
What happens if I don’t bleed my brakes? Your brake pedal will feel spongy and your braking force is dangerously reduced by air in the brake lines.
- About the Author
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John Cunningham is an Automotive Technician and writer on Rustyautos.com. He’s been a mechanic for over twenty-five years and has worked for GM, Volvo, Volkswagen, Land Rover, and Jaguar dealerships.
John uses his know-how and experience to write fluff-free articles that help fellow gearheads with all aspects of vehicle ownership, including maintenance, repair, and troubleshooting.