I've got a leak!! What to do?
The only thing that can be done on an old and crumbly roof is to tarp the area, using a tarp large enough to stretch beyond fromwhere the bottom of the valley is and run vertically up to the ridge of the dormer on the one side of the valley and up to the ridge of the main roof on the other side of the valley.
Since the tarps will be in place for a long time, buy the heavier density weave tarps which cost more, but won't disintegrate from the suns UV rays as quickly. Nail 2" x 4"s on the vertical perimeter edges and horizontally across the ridges with on nailed or screwed at the top of the valley, but dont nail that board any lower in the valley or it will cause more leaks.
The boards are to secure the tarp during the winds and to prevent billowing.
Alternatively, you could try to have a roofer apply a torch dowm APP modified bitumen over each valley and see if that seals tightly enough to the existing shingles to prevent significant water intrusion.
Any patch job done on the top of the shingles, is just temporary, at best.
Whatever you do, do not pour asphalt emulsion or plastic roofing cement over the valley, as it will dry up and when it cracks, it will be bonded to the existing shingles and further intensify the existing leakage. Plus, then you will have a hard time getting any roofer to even want to work on that gunky mess.