Brake Bleeding Issue - Mind Blown!! - NOT RESOLVED

morch_66479 · 2025-10-10T03:14:19+00:00

Hi All, This isn't my first brake bleed and have been successful int he past with my Motive bleeder kit. I’ve recently installed a 320mm Big Brake Kit (wi...

Anthony

Reminder - the clutch hydraulic activation system is also connected into the brake fluid system. This also needs to be bled properly.

morch_66479

Anthony thanks, my car is an automatic so shouldn't apply.

trevingle45_

Your post brought to mind a thought that's been rattling around in my head for some time. I have the 6 pot polestar brakes but I think the master cylinder is the standard unit. The question is,,, does an increase in caliper capacity require an equivalent increase in master cylinder capacity?

I am putting this thought out for consideration.

morch_66479

trevingle45_ with regards to my situation. The stock piston size is the same as the piston on BBK (it's an xr-5 Caliper, much beefier overall but same piston size)
Surprising I know!!

ramrod

How would air be allowed to enter the ABS pump?

Clamp the brake lines (all four) then pump the pedal.

This will indicate the master bypassing fluid

morch_66479

ramrod tricky as all my flex hoses are braided.

jamesinc

Volvo's take is this:

Image description

If the total sectional area of the caliper pistons increases, you'd need to increase the bore of the brake master cylinder to keep the same pedal travel. If you didn't, you'd expect more travel than previously to get the same amount of clamping load.

ramrod

Try bleeding each line going out of the ABS pump, at the ABS pump itself

Difficult to do without allowing air back in, and then re-bleed each caliper.

See if any difference can be felt

morch_66479

ramrod
This would be very a tricky task but it might be something worth doing alright :-)

rado

Just an idea , are you bleeding it by yourself ?

I ask as it sounds very similar to when I done mine and very frustrating . In the end I got my wife to sit in to pump the brake pedal while I ran around to do the bleeding . Worked a treat for some reason .
I had all 4 wheels off jacked up ( obviously) for easy access . I ended up throwing the bleeding kit in the bin .
Just saying, worked for me .

morch_66479

rado yeah so on the 5th bleed I actually carried this out with a friend who also has some mechanical knowledge and carried it out the old fashioned way. I've modified the original post to reflect this.
I though that would fix it for sure

Jamest5r_

How much brake fluid are you keeping in the reservoir when attempting this? I know it sounds stupid and no visual reason for it but it really needs to be literally at the top of the cap so you can't physically get anymore in and keep it at the max level at all times during the process, yes I know it shouldn't make and difference but with the clutch (I know you don't have one) and the brakes for some weird Volvo reasons used to work for me many times

morch_66479

Jamest5r_ Brake fluid levels during bleeding witha person were always between max mark and the neck of the reservoir
levels when using the motive kit were right to the top of the neck and the motive bleeder keeps it full full at all times.

Inski

Try reverse bleeding through the calipers.

Jamest5r_

morch_66479 Yep I understand, however the phenom we suffered with a mates 850R was even the max level wasn't enough it had to be full full to the brim, we tried doing it 5/6 times and 4 of us there all experienced, did our heads in as we kept get air for no reason, it was a last resort, maybe we got lucky and it was a coincidence but that's what we did and it worked 🤷

morch_66479

I think my next step might be to jack up and remove the front wheel and actually inspect the full mechanical operation of the calipers themselves - clamping, guides etc etc.
I'll prob do this at the time of swapping to a 302mm kit as my curiosity needs to know why im having issues.

Appreciate all the advice guys :-)

morch_66479

Jamest5r_
OK gotcha. I must also try this too just to be sure. Really appreciate the feedback ☺️

trevingle45_

Going thru the same issue. Back in the good old days I used a long length of tube fixed to caliper bleed nipple running back to brake fluid reservoir with outlet in fluid and cycled the fluid many times. When the tube was filled with fluid it became a sealed unit but able to vent any air in the reservoir. Time consuming but always worked without wasting fluid.

Elliot

I know this may sound silly, but are the calipers fitted on the correct side of the vehicle with the bleed nipples at the top of the caliper?

morch_66479

Elliot not silly at all, good to work through all potential options.
So yes that's all correct.
I think I found the issue yesterday though - check the next post that a plan on writing up this morning.