Bleed Brakes: DIY Guide & Tips

Hello everyone,

I desperately searched for information on how to properly bleed the brakes on a specific vehicle, like a VW LT 35 II with ABS and EBD, but unfortunately, I couldn't find anything.

Here's the problem I'm experiencing with my car: The braking performance is excellent; on the TÜV (German technical inspection) brake tester, the gauges reach their maximum values; the braking force is evenly distributed across all wheels on each axle. However, I can also depress the pedal fully if I apply enough force. The brakes also work well during normal driving, even with only a small amount of pedal travel. Nevertheless, the pedal pressure becomes harder after repeatedly depressing the pedal, which usually indicates air in the system.

That's why I changed the brake fluid and then bled the brakes a second time, using a brake bleeding device, including the clutch slave cylinder. However, the symptoms I described are still present!

Kann mir da jemand helfen? Sure, here's the translation:

'Thank you in advance.'


Rainer.

Okay, wait a minute... if the brake pedal becomes hard after repeatedly pressing the brake, isn't that more likely a sign of insufficient vacuum in the brake servo (BKV) rather than air in the system? Am I wrong about that?

Hi!
Actually, Matthias is right; a stiff-feeling pedal usually indicates a lack of vacuum. But maybe we've misinterpreted your description?!
I once had the same problem with a Golf IV 1.4 (ABS but no EDS). Despite using a filling device, the brakes felt "spongy." The solution was to reduce the pressure of the filling device... and, of course, to bleed the brakes again.

Best regards,
Dirk.

hello,
I also suspect a problem with insufficient vacuum, perhaps due to a slowly weakening vacuum pump or a leak in the line to the BKV (brake control unit). Replacement pumps are already offered by various manufacturers. I would recommend a vacuum pressure test for diagnosis.

Best regards,
Gerrit

Oops, that was quick! Thank you very much for your tips, I will check the vacuum issue immediately!

Rainer.

Hi,
'I couldn't depress the pedal fully, but it felt 'soft'. The cause was poor, 'rusty' brake pads. Rust had penetrated the lining material on the backing plate! The pads then became compressible, which manifested as a soft brake feel. Mine passed the technical inspection (TÜV) without any issues. After replacing the pads, the problem disappeared. These were original pads and had been in use for a maximum of 3 years.'

I also noticed some hardening (when the engine was running).
The cause is likely that the material between the plates doesn't fully expand immediately when compressed, and therefore becomes less compressible with repeated pressing. This effect becomes more noticeable with repeated 'pumping' actions. If you leave it undisturbed for a while, it becomes as soft as it was initially.

That has nothing to do with air.

You can't really see that from the outside. I'll check if I still have them and then take a picture.

Hello Garth,

Thank you for your explanations! The brake pads are new (4,000 km) and were replaced after there was occasional 'rubbing' when braking; it sounded like metal on metal. When I removed the old, non-rusted pads, I noticed that all four outer pads had cracks; the pad thickness was still very good, about 10 mm each! After the replacement, the rubbing and scratching stopped, but there was this strange spongy feeling in the pedal. However, the braking performance is perfect, as confirmed during the subsequent TÜV (German technical inspection) test.

Thank you very much to everyone for the helpful tips!

Best regards,

Rainer.

(VW LT 35 II camper van; 2.5L TDI 75 kW)

Hello.

I'm experiencing the same problem with my Golf 4 TDI! The braking performance is the same after replacing the front brake pads, the axle is balanced, and the ABS is working perfectly.
But the brakes feel really spongy!

I initially suspected a problem with the vacuum supply, but the entire pneumatic control system is working! I think the brakes take priority; if there's insufficient vacuum, the VTG (Variable Turbine Geometry) and AGR (Abgasrückführung - Exhaust Gas Recirculation) systems will also stop functioning.
A 'spongy' feeling usually indicates too much vacuum, right? What is the function of the check valve in the vacuum line to the BKV (blower control valve)? Does it regulate the vacuum?

Hmm, if I read Brezelmann's post, it seems like the pressure pushing back on the piston might have been too high! And that could cause the same effect as having too much pressure on the filling device.

To everyone, could that be possible?

Best regards, Ferdi.

Here's the picture I promised, even though the problem at Karmann likely has different causes. You can clearly see how the blade fits sideways into the pads. The pad material is then soft in that area (see above). The pad thickness of 3-4 mm was still acceptable.

deleted

Hello,

Thank you again for the tips and the picture! I will try bleeding the system again tomorrow with reduced pressure before replacing the vacuum pump and vacuum hoses. So far, I've been using around 3 bar, so I'll try 1.5 bar and then adjust my approach based on my experience.

See you later.

All the best.

Rainer.