Brakes locking up during the race

[Micke68]

Has anyone had their brakes actually almost get to the point of locking up during a race? This past Saturday we were really fast during practice and the heat race and the brakes were functioning properly. At the start of the feature, which was about 1.5 to 2 hoours after the heat race, the car was good, but quickly after that they started to drag some. On lap 7 there was a caution and then upon the restart the issue progressively got worse. When my son came off of the track after the 35 laps we lifted the front of the car and could not physically turn either one of the front wheels. We then lifted the rear and could turn the rear wheels slightly. I then went into the car and tried to work the brake pedal and it wouldn't depress at all. So I am pretty certain that it was a master cylinder failure. However I would like to know what caused it and if anyone else has ever experienced such an issue and what was found.

Thanks,
Mick

[igotmylegend]

Look to see if the push rod came out of its seat in the master cylinder.

[VMS Motorsports]

Good point Vince.
Also try bleeding the brakes.
Its most likely the Master Cylinder, but best to try the free stuff first

[Micke68]

The brakes were bled the night before the race. The thing I forgot to mention and may be of interest is that about 30 minutes after the race everything had freed up and the car rolled fine. Yesterday morning after we unloaded the car in the shop i rolled it back and forth and worked the pedal by hand and it functioned properly, so I don't believe that it is the shaft, but I will take a close look tonight. Thanks for the suggestions.

Mick

[Winterracing]

is it a heavy braking track?  I have seen that the brake fluid can get so hot that it will lock the brakes up.  What type of fluid did you use?  I personally use the willwood stuff and have never had a problem

Dan
www.winterracing.net

[racerrad8]

Make sure the brake pedal/pushrod have some play in the master cylinder. Loose the 7/16" nut & shorten the pushrod slightly if needed.

During longer events the heat in the master cylinder can cause the aluminum piston within the master cyliner to expand thus letting the pedal stop keep the brakes slightly applied by not letting the piston return all of the way to the snapring.

When this happens it covers the transfer port of the cylinder and holds the brakes on and only the front since the rears only work when pressure is applied by the by the force of the piston and the pressure build up in the front brakes.

Randy - RPM

[Micke68]

Thanks for the info guys! It's a 3/8 mile track and the braking is not hard, just enough to get it turned and set. We use Wilwood 600, so I am nearly certain it wasn't a fluid issue. It sounds much like what Randy is suggesting since the pedal and brakes became free after about 30 minutes. We will investigate more tonight and see how much play there is. I am concerned this might occur again with this master cylinder so we are going to change it since I don't want to take that chance.

Thanks,
Mick