69 Brake Auto-Engagement Issues
 

Hello all,
I am aware that there are numerous postings/threads about brake issues however I have not come across one that describes the situation that is occurring with my 69.

Short Description: After driving for approximately 6 miles with normal braking situations, both front brake calipers will not release i.e. lock up, drag, and will not release until the car is parked and off for ~30 min.

Relevant Vehicle Components:

• OEM style front disc: single piston, sliding caliper
• OEM style drum in rear
• OEM style dual master cylinder (MC) w/ power brake booster, proportioning/combination valve
• Appropriate brake line sizes to front and rear
• DOT 3 fluid, clean

Longer description of issue: For the first ~6miles of driving, the braking system acts normally. In addition the car is able to "freewheel" i.e. roll without resistance, when shifted into neutral at speed. At about 6 miles the car starts to slow itself down if shifted into neutral at speed. This gets exponentially worse to the point that the vehicle will stop itself in neutral, without the use of the brake pedal. Limping the vehicle home and jacking up one side reveals that the tire will not spin (as expected). This occurs on both front sides. As expected the brakes are smoking hot, depending on distance back to the house from the time the issue starts. The calipers are stuck in the "clamped" position.

Investigations performed:

• Removed both front calipers, greased slide pins with appropriate lubricant
• No visible leaks from hydraulic system
• Bled front brakes several times
• Ran the car stationary and pumped brakes 50 times to simulate braking without increasing pad/disc heat. This did NOT cause a lockup situation and was mostly an attempt to understand the role of heat

Next course of action: Determine whether the pushrod/shaft from the booster to the master cylinder is too long and putting "pre-load" on the system. Get that brakes to lock up and jack vehicle up. Verify no rotation of front wheels and then unbolt MC from booster to provide separation of the push rod from the MC. If the wheels then rotate, the length of the push rod may be the issue. My only concern with this test is the fact that there appears to be no pre-load or brake dragging when first driving the car. This would mean there would need to be something that changes over the course of driving, perhaps thermal expansion of the brake fluid, that creates the issue.

What do you all think? Have you experienced anything like this?

1969 Camaro | COPO Clone | V8 350 SBC 5.7L LT-1 | Quick Fuel Holley 4 Barrel DFT Slayer | Doug's D368 Headers | MSD Ignition Street Fire HEI Distributor | Disc Front | Drum Rear

You might disconnect the rod from booster-to-pedal and shorten it by about min2 turns and try again.
If better, but not perfect, add another thread to adjustment.

Was something changed on the brake system? If not this is almost always the hose going bad at the front calipers. The hose degrade on the inside but since there is thousands of pounds of pressure when braking that will push through the collapsing hose. But you need to release the pressure the hose will only let the pressure bleed off so fast as the pressure drop it bleeds off slower and slower. I have had customers tell me they waited 10 hours only to have it bind up again after they used them.

I would suspect any hose more then 8 years old but I have seen them fail at 5. Any brake job you do and they are more then 7 years old should include new hoses. I do not mean to say all hoses fail. A few years ago I did my 67 and I think the rear hose to the rear end was original to the car. Just that 80% of the time that a car showed up with sticking brake it was cause of a hose and 18% or so it was rust in the brakes system. The rest where old ball stuff like a stuck slider or contaminated brake fluid, a damaged seal that was binding. I have never had to adjust anything at the master unless there was new aftermarket parts involved and you where messing with the design of the brakes.