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Pressure Vs Reverse bleeding the brakes

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Hi guys!

As I am approaching the need to perform a brake bleeding job on my car I would like to leave behind the manual/pedal technique and use something different.

I have read a little bit online and it seems that the pressure bleeding method and the reverse one are the best.

I have always used the vacuum method but a lot of air enters the tube in the purge-nipple vicinity and it makes my life harder to see when the fluid is bubble-free.

So... Here are my doubts:

Pressure bleeding or reverse bleeding?

Pressure bleeding seems easy and effective.

Reverse bleeding, in theory, should be a little better as it force air up, which is its natural direction (not as pressure bleeding, which forces air down).

I am also a little worried with reverse bleeding because I feel I could 'push' some dirt in the brake calipers vicinity towards the rest of the system, so maybe I should first flush the system with new fluid using the vacuum method and then the reverse!

What do you think? What is the Pro opinion?

I have attached some pictures of REVERSE/PRESSURE/VACUUM bleeding for reference.

I have looked for brake bleeding methods on the forum/HPA channels but I can't seem to find anything (hey guys, maybe it could be an idea for a video? :) )

P.s. my car is a 2018 GT86 with Brembo brakes.

Attached Files

My concern with reverse would be both contamination and that it's usually the bleeder, not the brake line that is the high point on the calliper so you may never get the air out.

Hi Slides, excuse me but English isn't my first language... I am not sure of having understood 100% what you have written... Why shouldn't I be able to get all the air out of the braking system by using the reverse bleeding method?

As you said, air rises.

Most brake calipers are designed and fitted with the brake bleeder at the highest part of the caliper and the fitting where the fluid enters the caliper is at a lower point. This means that reverse bleeding is unlikely to remove all the air as the movement of the fluid is unlikely to draw all the air out of the caliper. There may be an arguement for the method if it's known there is no air in the caliper and one merely wishes to flush old fluid out and replace it with fresh.

The problem with the vacuum method is that all the seals are designed to retain pressure in the system, and are not intended to prevent air entering the system, past the seals, when the pressure differential is reversed - this is primarily an issue with the master cylinder(s), and wheel cylinders if drum brakes are used on the vehicle. You've already found there's a problem and, if you look at the seal designs, it should be clear why that's happening.

I would suggest either low pressure bleeding applied at the master cylinder(s) and bled at the calipers, as you're considering, or continue the old fashioned manual bleeding where the pedal is depressed to pump fluid through the system. I've used the latter for decades without any problems, either with a second person or by using bleeders with check valves in the valves or bleed hose - but others may have different preferences.

There is one other thing you may need to consider, and that's the 'shelf life' of the brake fluid. It's agroscopic (absorbs moisure), with the higher BP fluids usually being more susceptible to this, so one will usually try to use it as soon as possible after use, and some pressure bleeders may hold more than can be readily used.

Hi Gord! First of all, I really want to thank you for the deep reply, very detailed!

You made me think about aspects that I had not focused on. I am really happy to discuss this with you.

In particular, that fact about the position of the bleeder valve vs the fitting where the fluid enters the caliper.

It seems that the manual method and the pressure one are the best! I know the first one and would like to try the second!

I know that brake fluid absorbs moisture and I always keep my fluid bottles sealed until it's time to flush it!

I have just looked online at some pressure bleeder and they come at a great price.

What do you mean when you say "some pressure bleeders may hold more than can be readily used"?

Just to be sure, I would like to ask you a thing:

My car has about 1/1,1 Lt of brake fluid. I found that a lot of pressure bleeders come with a 2,5 Lt capacity. Can i fill my pressure bleeder with 1,5 Lt (as my Motul brake fluid bottles come in 0,5 Lt packages) and flush/bleed the system? Or do I have to fill the pressure bleeder up to the top level? I hope that my English is clear enough! In the case it isn't, I will try to change the words used :)

Sorry, that was unclear, some commercial bleeders may have left-over fluid, which may not be used for some time and have a (small) risk of absorbing moisture. I expect you can just use the 1.5 litres, possibly using the excess up as well, to help the flushing. Just make sure it doesn't empty the reservoir or you'll have to start again - did that as a 'prentice many years ago.

Don't worry about your english, it's VERY good - my Italian is limited to, maybe, three or four words.

Thank you Gord!

Italian is not as useful as English, so you aren't losing anything :D

Jokes apart, you have been really helpful with my brake bleeding solution :)

Browsing on the internet, I have found 2 main devices for power bleeding. As you can imagine, I would really like to know your opinion!

The first system (I have uploaded a picture for reference - System A) is made by different manufacturers (Sealey is just the first one that appeared after me googling it) and has a little gauge to show the pressure in the system. They generally recommend about 10~15 psi (0,66~1 bar) for bleeding.

The second system (I have uploaded a picture for reference - System B) is only made by Phoenix Systems and can be used for vacuum, pressure, bench and reverse bleeding (it depends on the fittings/tubes configuration used). When used for pressure bleeding, It doesn't have any gauges (so there is no way to know the pressure inside the circuit - is this a problem in your opinion? Would I risk to do damages in your opinion?).

Here is a link to their website -> https://www.brakebleeder.com/product-category/brake-bleeders/

Which style of tool would you like most?

Attached Files

I haven't tried either, but I'd be inclined towards the first as I'm not 100% convinced of the strength of some makes of plastic reservoir and the gauge may prevent over-pressurising and possibly exploding the reservoir - they're designed for just holding the fluid, not normally for being pressurised.

The second is potentially more versatile, but if you don't plan to use the other options it's wasted.

Anyone else to comment?

On European cars, pressure bleeding is the correct method for bleeding brakes, particularly with some of the nitrogen assisted ABS units such as Mercedes SBC system.

One of the dictating factors is the reservoir design, as many will not suit pressurising. A lot of the Toyota masters had pop on rubber caps, but as you have an 86, your car has the screw type reservoir. I would recommend pressure bleeding at a lower pressure (0.6bar) as the only thing that retains your reservoir onto the master cylinder is a singly pin.

Thank you Gord and thank you N8B!

I will try the lowest pressure that allows me to bleed the system and... strategically place a set of cotton rags in the proximity of the reservoir, just in case.

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N8B I have seen that you are 'concerned' about pressuring too much the reservoir. Would you exclude a bleeding tool such as System B for the lack of a gauge? (A few posts ago I uploaded two pictures of two different styles of bleeding tools)