BMW E12 V8 Racecar Build

[Navigator_E90]

Brake System Gremlin – The Most Time-Consuming Frustrating Issue​

This was not necessarily the biggest problem encountered during the build, but without question the most difficult and time-consuming gremlin to diagnose and resolve.

During the first full race at Red Star, the brake pedal began to feel progressively spongy. The issue was noticeable even in the pit area: with steady pressure, the brake pedal would slowly sink further toward the floor. On track, the problem worsened to the point of being genuinely concerning.

The only temporary solution was to pump the brake pedal repeatedly before each braking zone, which restored pedal firmness momentarily. This obviously resulted in significant time loss, but completing the race was the priority in order to gather track data.

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Initial Diagnosis & Failed Assumptions​

The symptoms strongly suggested air in the brake system. However:

• The system was bled multiple times on race day
• No air was found during bleeding
• No improvement was observed

Back at home, all brake lines were inspected again. A faulty master cylinder was suspected and subsequently reconditioned. This made no difference.

One notable observation during testing:

• With the engine off, the brake pedal felt firm
• Once the engine was started, the pedal immediately became spongy

Brake fluid type was ruled out — race-spec fluid was used throughout and tested again with no change.

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Component Substitution & Isolation Testing​

The car was then taken to a race car specialist. His immediate diagnosis was again the master cylinder. To eliminate doubt, a known-good master cylinder from a running race car was fitted. The issue persisted.

A small wet area was found on a brake line, raising hopes that this was the air-entry point. The line was repaired, but once again no air was ever found during bleeding, and the pedal behavior remained unchanged.

At this stage, the braking system consisted of:

• Stock E90 master cylinder and Booster
• Stock E63 brake calipers and rotors
• Braided brake lines throughout (installed from the start)
• New new hard lines fitted during testing

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ABS System Investigation​

Further research uncovered a case where similar symptoms were caused by a faulty ABS pump. With no other clear direction, the ABS unit was replaced — with no improvement.

Additional research suggested that some ABS systems require electronic bleeding via diagnostic software. The existing diagnostic tool did not support this function, so a second tool was sourced and the ABS bleeding procedure completed successfully.

Result: no change in pedal feel.

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Systematic Line Isolation​

At this point, a full isolation test process was started:

• Master cylinder outlets blanked → no pedal fade
• Lines blanked immediately after ABS pump → no pedal fade

Rear circuit testing:

• Right rear only → no fade
• Left rear only → no fade
• Both rear circuits connected → no fade

Front circuit testing:

• Each front circuit tested individually and front combined → no fade

However:

👉 The moment all four circuits were connected simultaneously, the pedal fade returned.

This occurred despite:

• New brake lines
• Braided hoses
• Multiple master cylinders
• Brake booster replaced
• New Vacuum lines - and vacuum pressure checked
• New ABS units & followed bleeding process
• Repeated bleeding cycles

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Root Cause Identified​

At this point, the fault was finally identified.

I’ll explain exactly what caused this behavior and how it was resolved in the next post — as it turned out to be something extremely easy to overlook, yet almost impossible to diagnose without systematic isolation.
Would like to hear your ideas and opinions on what could have caused this, before I reveal the problem

 

[Benji]

The suspense is killing me!

Could it be some sort of short circuiting between the front/rear circuits, possibly a crack in a connection block or the separator seal thingie in the master cylinder?

 

[Navigator_E90]

Nope.
There is 2 braded hoses marked V & H from the Master Cylinder front and rear circuits directly supplying the ABS pump - then from the ABS pump hard lined to each wheel VL VR, HL HR - marked on the ABS pump for every wheel

Tip: it was a hydraulic problem

 

[Navigator_E90]

Club Racing Context & Regulations​

With the previous E90 build, I competed in the BMW M Performance Race Series, a BMW-only championship as the name suggests. The primary technical regulation is that the BMW must be powered by a BMW engine. Importantly, the engine does not need to correspond to the original model year of the chassis, which makes the E12 with the E63 V8 power plant combination fully compliant.

The championship uses a time-based classification system rather than strict technical regulations. Each class is assigned a minimum lap time per circuit. For example, at Zwartkops Raceway, a Class D car has a minimum lap time of 1:12.0 (2025 reference). If a car laps faster than the class limit — even by 0.01 seconds — it is automatically promoted to the next faster class "C".

This system allows a wide range of vehicle specifications with minimal technical restrictions, aside from mandatory safety requirements.

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Race Format Changes 2026​

Previously 2025 and prior, due to the number of competitors, the series was split into two race groups:

• Race Group 1: Classes A, B, and C
• Race Group 2: Classes D, E, and F

Each group included both turbocharged and naturally aspirated cars in all classes

For 2026, the format was revised while still maintaining two race groups:

• Race Group 1: Turbocharged cars only (Classes A, B, C, and D)
• Race Group 2: Naturally aspirated cars only (Classes A, B, C, and D)

This change aims to group cars with more comparable power delivery characteristics, improving race flow and on-track safety.
Its going to be an very interesting race year!

 

[Navigator_E90]

Check the link below, 1st race at RedStar Raceway
Car 57 with the wing is a turbo car - see how he pulls away in the first straight

 

[Bugger]

Check the link below, 1st race at RedStar Raceway
Car 57 with the wing is a turbo car - see how he pulls away in the first straight

Geez the sound of that engine is intoxicating. And it looks like so much fun.... Just great, all of the information you've provided, the insight, really is a great thread this

 

[evnmopwr]

Check the link below, 1st race at RedStar Raceway
Car 57 with the wing is a turbo car - see how he pulls away in the first straight

Jissis !!!