Front braking power suddenly gone

It might be best to not open the pressure control valve and only bleed at the calipers.    There could be air trapped in the pressure control valve at this point.  Maybe just gravity bleed the front calipers one at time and test the car again.  Be careful to not let the reservoir run low.  Bleed the bottom first and finish at the top bleeder.

If the problem still happens after bleeding as described above then maybe focus on the brake master cylinder.  Its seals may be tired.  One of the symptoms of bad seals in a master cylinder is needing to pump more than once to get pressure.  Another symptom is fluid leaking out the back.  There may be enough wiggle room to unbolt the master while leaving the brake lines connected and check for leaking fluid at the back.

It sounds like your shuttle valve is working correctly.

Also, you can test the brakes after bleeding while the front wheels are still off.  With the ignition switch turned to RUN and the engine off just press on the brake pedal.  Hopefully you get a firm pedal but you may get nothing or somewhere in between.  Watch for the brake warning light and listen for the corresponding clunk from the shuttle valve.  

@stevebuchanan posted:

It might be best to not open the pressure control valve and only bleed at the calipers.    There could be air trapped in the pressure control valve at this point.  Maybe just gravity bleed the front calipers one at time and test the car again.  Be careful to not let the reservoir run low.  Bleed the bottom first and finish at the top bleeder.

If the problem still happens after bleeding as described above then maybe focus on the brake master cylinder.  Its seals may be tired.  One of the symptoms of bad seals in a master cylinder is needing to pump more than once to get pressure.  Another symptom is fluid leaking out the back.  There may be enough wiggle room to unbolt the master while leaving the brake lines connected and check for leaking fluid at the back.

It sounds like your shuttle valve is working correctly.

Thanks! In my understanding the master brake cylinder cannot be the cause - a failure there would not trigger the brake warning light, right? And I am not loosing brake fluid.
Looking forward to my next test drive, but the conditions here in Austria are not right for now.

best wishes.........Eugen

Eugen -

I would think the master CAN be the cause, and your pressure differential warning switch is correctly sensing, and indicating an imbalance in brake pressure between the two circuits….

Rocky

@rocky posted:

Eugen -

I would think the master CAN be the cause, and your pressure differential warning switch is correctly sensing, and indicating an imbalance in brake pressure between the two circuits….

Rocky

I agree

Thanks, valuable information, I will renew/refurbish my brake master cylinder too!

Short update - issue solved! I changed the master cylinder, the braking is now perfect! What a difference! The brake warning light stays off too :-)
Thanks for all the help here!

Eugen

I think it is fairly safe to diagnose your intermittent problem was due to a worn rubber MC cup, allowing fluid to internally bypass the cup instead of being pushed into the front brake line.

something that the visual inspections and air bleeding would always fail to discover/fix.

Glad you managed to solve it

Larry

Thanks, and yes, it really was a strange behaviour - working normally and then sometimes suddenly no pressure in one brake circuit! I should have replaced the MC immediately when I saw the sticking calipers and the rust in the bores!

Eugen

on a side note . . . My take away was that You noticed a braking deficiet, BUT did NOT notice the differiantial pressure WARNING lamp (at first).    Thus giving me a fuzzy "OK" feeling about my switch being disabled.

Yeah, the first time the braking power was gone I was quite busy trying to find a place to avoid the oncoming truck on a small back road. :-)

Can you share what master cylinder replacement you used and where you purchased it?  A pic would help.  thanks

Sure, I bought it off ebay.com, the seller was PIM. See picture installed. It was really a direct replacement, I bench bled the MC before and after the installment I did not have to bleed the system at all - I immediately had a firm pedal and the brake performance is great (I will bleed the system anyway in the next days just to be sure).

Fiat Ducato use the same brake master cylinder.( cheap)

or C+C Parts , Baarlo, The Netherlands rebuild them an use a brass sleve

€350.-

Simon

What am I missing on 3 lines coming from that Master?  Mine has two.  Where is that middle line going?

Patt, it is an optical illusion.  There are only two lines coming out of the brake master cylinder.  The extra line is going to the Pressure Control Valve that is located beneath and behind the brake master cylinder in the above photo.  The Pressure Control Valve is mounted to the fender wall.  The silver bolt head on the master just happens to be near the mystery brake line in the photo.

Hi,

I use  a 308 RHD brake master cylinder buy at Superformance.co.uk same as original and works very well :  # 30806095 for 269.95 £

@Seethaler posted:

Yeah, the first time the braking power was gone I was quite busy trying to find a place to avoid the oncoming truck on a small back road. :-)

even though front brakes were lost . . . would you think the rear were still 100% of their capibility , (proabably with more pedal travel)

hard so say, the first time it happend it felt like almost all brake power was gone. The pedal travel was a lot more. On my other test drives I just double pumped when the red warning light in the dash was on. :-)

I also bought a master cylinder from Ferrari 308 right-hand drive strictly conforming to the original one at TWS-Parts for 263 €