Bleeding Brakes ?

[SCguy]

I bled my brakes yesterday, but am not sure that I've got all the air out.

How can I tell?

 

[RonMacPherson]

Push the master cylinder down as far as it will go, lock it in place with a rod of some sort, propping against the seat, wheel, etc. Go away over night and come back the next morning to see if the pedal has dropped any.

Or get some kind of power flusher to bleed the brakes.

 

[Deleted member 7471]

A good firm pedal is the non-scientific test I've always used. If it feel 'squishy' with a lot of pedal travel, there's probably air in the system. If you've got a vacuum booster, it may be a little bit harder to tell.
Maybe someone has a more objective measure...

 

[Don Elliott]

Did you "bench-bleed" the master cylinders before starting ?

 

[Geo Hahn]

If you can get firmer pedal by repeatly pumping the brake that suggests there is air in there (the pumping compresses the air to a point where the pedal gets firmer).

 

[KVH]

Although I've bled brakes and clutch cylinders many, many times, usually with a friend pushing the pedals, I have to confess that, on a TR4 at least, I've never understood "bench bleeding."

What does it do and how is it done?

Won't the air come out just bleeding from the bleed screws with a hose into a clean bottle of fluid?

thx

 

[piman]

Hello KVH,


"Won't the air come out just bleeding from the bleed screws with a hose into a clean bottle of fluid?"

Yes it will, but if you start with an empty system, it can take some time to get the fluid through.


Alec

 

[tdskip]

Larry,

I've found using a Gunson Eezi-bleed works best for me - makes it a one person job and very easy to make sure no air it trapped.

 

[Don Elliott]

I Googled "BENCH BLEEDING BRAKES" and found these among many explanations

https://www.misterfixit.com/brakbld1.htm

https://www.vtr.org/maintain/brake-bench-bleeding.shtml

 

[TR3driver]

Simple rule of thumb :

If you can't tell they have air in them, they don't /bcforum/images/%%GRAEMLIN_URL%%/smile.gif

IMO, the point of bench-bleeding the MC is to get the working cylinder area as full of brake fluid as possible; only to make bleeding on the car easier. With no fluid in there at all, a full stroke of the MC may not generate enough pressure to force fluid through the system, making bleeding difficult or impossible. If this turns out to be the case and the MC is already on the car, you can solve the problem by bleeding right at the MC outlet (with rags stuffed underneath to preserve your paint). Just loosen the fittings as someone depresses the pedal, then tighten them before releasing.

BTW, I disagree with both Geo and Ron about the symptoms of air in the system. The air will compress instantly and not change overnight; so if the pedal falls overnight it means there is a leak somewhere. (This is a classic symptom of a bad return valve in the MC.) And while being able to "pump the brakes up" may result from air, it also results from the normal travel in the calipers and cylinders (plus lots of other possible problems, including stuck pistons and poor adjustment).

If you can lock all 4 wheels with a single stroke of the pedal, then there is not enough air in the system to cause problems. And any tiny amount remaining will work it's way out as you drive.

 

[Don Elliott]

For what it's worth, my brakes worked fine for 11 years with silicone fluid, then I drove to VTR in Colorado in 2001. Altitude 12,095 feet through Estes pass. The air that I thought I never had in the system, "boiled out", like a diver who comes up too fast, he gets the bends. I had to pump my brake pedal two or three times to come to a stop. Bob Palmer from Ohio helped me bleed them to get rid of this high altitude air. Since then, they work super.